Choose the group in which all numbers belong to the same category:

Correct — A.
Rule: every number in the group must share one property.

2, 4, 6, 8: each is a multiple of 2, so all four are even.
Every other group mixes odd and even numbers, and each starts a times table (3, 6, 9 · 5, 10 · 7, 14) that its last numbers break.

The group that holds together is 2, 4, 6, 8 → option (a).

Why the others are wrong

• (b)
  3, 6, 9, 11 — 11 breaks the multiples of 3: after 3, 6, 9 the table needs 12. The group also mixes odd numbers (3, 9, 11) with the even 6.
• (c)
  5, 10, 13, 21 — 13 and 21 break the multiples of 5 that 5, 10 begin. The group also mixes odd numbers with the even 10.
• (d)
  7, 14, 26, 38 — 26 and 38 are not multiples of 7, though 7, 14 start that table. The group also mixes the odd 7 with three even numbers.

A same-category item asks for the group whose members all pass one test. Try the plain tests first: even or odd, prime, multiples of one number, squares or cubes.

The wrong groups here look like times tables for their first two or three numbers and then break the pattern at the end. Check the last number of every group before choosing.

Key facts

• Even numbers are the multiples of 2: 2, 4, 6, 8 …
• The first multiples of 3 are 3, 6, 9, 12, so 11 breaks that table.
• The first multiples of 5 are 5, 10, 15, 20.
• The first multiples of 7 are 7, 14, 21, 28.

Common traps

• Stopping after the first two numbers of a group, where every group looks regular

No directly related past PYQ was found.