When a particular number is subtracted from each of 7, 9, 11 and 15, the resulting numbers are in proportion. The number to be subtracted is -

Answer: Option C

Let the required number be x
Then,
$$=\frac{7-x}{9-x}=\frac{11-x}{15-x}$$
⇒ (7 - x)(15 - x) = (11 - x)(9 - x)
⇒ 105 - 22x + x² = 99 - 20x + x²
⇒ 2x = 6
⇒ x = 3