Which of the following are in continued proportion?

Think it through first, then pick an option.

Answer and explanation

Correct answer: d 4, 6, 9

6² = 36 = 4 × 9; the condition holds.

Why the other options are wrong

a 1, 2, 8: 2² = 4 but 1 × 8 = 8; the condition fails.
b 2, 3, 9: 3² = 9 but 2 × 9 = 18; the condition fails.
c 8, 12, 16: 12² = 144 but 8 × 16 = 128; they have a constant difference, not a constant ratio.