With a 16:9 picture aspect ratio, display resolution 1080p means -

Correct — option (2), 1920 × 1080 pixels.

A resolution is written as width × height. In the name 1080p, the number 1080 is the height in pixels, and the "p" stands for progressive scan.

The aspect ratio 16:9 fixes the width. Divide the height by 9, then multiply by 16:
1080 ÷ 9 = 120
120 × 16 = 1920

So a 16:9 picture that is 1080 pixels tall is 1920 × 1080 pixels.

Wikipedia's 1080p article (October 2021 revision) describes the mode as 1,920 pixels across the screen horizontally and 1,080 pixels down the screen vertically. It adds that 1080p is often marketed as Full HD or FHD.

The idea to remember: the number in a name like 1080p is the height; the aspect ratio gives the width.

Why the others are wrong

• (1)
  1080 × 1080 pixels — Here the width equals the height, so the ratio is 1080:1080, which is 1:1, a square frame.

  A 16:9 frame must be wider than it is tall: its width is 16/9 of its height, about 1.78 times. For a height of 1080 pixels that width is 1920, not 1080.

• (3)
  720 × 1080 pixels — Dividing both numbers by 360 gives 2:3, a frame taller than it is wide, the opposite of widescreen 16:9.

  The number 720 recalls a different mode, 720p. Wikipedia's 720p article gives it as 1280 × 720 pixels, also 16:9: there 720 is the height and 1280 the width.

• (4)
  3840 × 1080 pixels — Dividing both numbers by 120 gives 32:9, a frame twice as wide as 16:9 for the same height.

  The width 3840 belongs to 4K UHD, 3840 × 2160, which Wikipedia's 4K resolution article calls the dominant 4K standard in television and consumer media. That frame doubles both of 1080p's dimensions and stays 16:9.

A digital picture is a grid of pixels. Its resolution is written as width × height: the number of pixels across, then the number down.

The aspect ratio is width compared with height. A 16:9 frame is 16 units wide for every 9 units tall, so its width is about 1.78 times its height.

Video modes such as 720p, 1080p and 2160p are named by their height. Wikipedia's 1080p article glosses the letter p as progressive scan, that is, non-interlaced.

Its 1080i article gives the i as short for interlaced: 1080i also has 1,080 lines, drawn as alternate odd and even lines.

Key facts

• 1080p is 1920 × 1080 pixels in a 16:9 frame; Wikipedia says it is often marketed as Full HD or FHD.
• 720p is 1280 × 720 pixels, also 16:9; Wikipedia's 720p article calls it HD ready or standard HD.
• 4K UHD is 3840 × 2160 pixels, 16:9: twice 1080p's width and twice its height.
• 1920 × 1080 = 2,073,600 pixels, about 2.1 megapixels; 3840 × 2160 = 8,294,400, four times as many.
• In a name like 1080p, the number is the picture height in pixels and p means progressive (non-interlaced) scan.

Common traps

• Order of the numbers: a resolution is width × height, so 1080p's 1080 is the second number, not the first.
• The number 720 points to 720p, but 720p is 1280 × 720; a width of 720 with a height of 1080 is a 2:3 upright frame.
• 3840 is the width of 4K UHD (3840 × 2160); paired with a height of 1080 it makes 32:9, not 16:9.

UnlockIAS compared this question with questions from other RAS Prelims papers and found none similar enough to link.

• practice — not a real PYQ

  A display mode with a 16:9 aspect ratio is 720 pixels tall. How many pixels wide is it?

  • (a)960
  • (b)1280
  • (c)1440
  • (d)1920
  Answer(2) — 720 ÷ 9 = 80, and 80 × 16 = 1280, the 720p frame of 1280 × 720. Option (1) is the width for a 4:3 frame (720 × 4 ÷ 3), option (3) gives a 2:1 frame, and option (4) is the 16:9 width for a height of 1080.
• practice — not a real PYQ

  How many times as many pixels does a 3840 × 2160 picture have as a 1920 × 1080 picture?

  • (a)2
  • (b)3
  • (c)4
  • (d)8
  Answer(3) — the width doubles (1920 to 3840) and the height doubles (1080 to 2160), so the pixel count grows 2 × 2 = 4 times: 8,294,400 ÷ 2,073,600 = 4. Option (1) counts only one dimension doubling, option (2) matches neither dimension, and option (4) would need a third doubling.