What should come at the place of question mark? : 3, 6, 18, 108, ?

Correct — A.
Rule: each term is the product of the two terms before it.

3 × 6 = 18 ✓
6 × 18 = 108 ✓
18 × 108 = 1944

Check: 18 × 100 = 1800 and 18 × 8 = 144
1800 + 144 = 1944 → option (a).

Why the others are wrong

• (b)
  648 — 648 is 108 × 6, repeating the last multiplier. The multipliers run × 2, × 3, × 6, and the next is × 18.
• (c)
  216 — 216 is 108 × 2, restarting the multipliers. The rule multiplies 108 by the term before it, 18.
• (d)
  864 — 864 is 108 × 8, a multiplier nothing in the series produces. The next multiplier is 18, the term before 108.

When a series grows faster than a fixed multiplier explains, compute the ratios: 6 ÷ 3 = 2, 18 ÷ 6 = 3, 108 ÷ 18 = 6. The multipliers 3 and 6 are themselves earlier terms.

That points to the rule: each term is the product of the previous two, 3 × 6 = 18 and 6 × 18 = 108. So the fifth term is 18 × 108, a jump of 18 times.

Key facts

• 18 × 108 = 1944.
• The ratios of 3, 6, 18, 108, 1944 are × 2, × 3, × 6, × 18.
• In a product-of-two series the ratio itself keeps growing.

Common traps

• Repeating the last multiplier, × 6, and choosing 648
• Looking for a fixed difference or ratio in a series whose ratio keeps growing

No directly related past PYQ was found.