Select the number from among the given options that can replace the question mark (?) in the following analogy.
56 : 15 :: 76 : 21 :: 81 : ?

56 : 15 :: 76 : 21 :: 81 : ?

we see that 56: 15

= (5 × 6) ÷ 2 = 30 ÷ 2 = 15​

again 76: 21

= (7 × 6) ÷ 2 = 42 ÷ 2 = 21​

81:?

= (8 × 1) ÷ 2 = 8 ÷ 2 = 4

Thus, the correct answer is (a).