Correct Option: D

Let us assume the number is P.
According the question,
three -seventh of a number = 3/7 x P
one -fourth of three -seventh of a number = 1/4 x 3/7 x P
Two-fifths of one -fourth of three -seventh of a number = 2/5 x 1/4 x 3/7 x P
⇒ 2/5 x 1/4 x 3/7 x P = 15
⇒ 2 x 1 x 3 x P = 15 x 7 x 4 x 5
⇒ P = 15 x 7 x 4 x 5 / 2 x 3
⇒ P = 5 x 7 x 2 x 5
⇒ P = 350
∴ P /2 = 350/2
⇒ P /2 = 175