Option (a) ⇒ We express number as the sum of the product of its digits with their respective place values

1000 a + 100 b + 10 c + d is the generalised form of four digit number abcd but we want generalised form of four digit number abdc

Option (b) ⇒ 1000 a + 100 c + 10 b + d is the generalised form of acbd. Hence this option is incorrect.

Option (c) ⇒ 1000 a + 100 b + 10 d + c is the generalised form of the four digit number abdc. Therefore this option is correct.

Option (d) ⇒ a x b x c x d is simply the product of a,b,c,d .It is not a number it is the product of digits of the number. Therefore (d) option is incorrect.