In the given network of AND and OR gates f can be written as:

A

$$X_0X_1X_2......X_n +X_1X_2......X_n + X_2X_3......X_n .......... X_n$$

B

$$X_0X_1 + X_2 + X_3 + ....... +X_{n - 1}.X_n$$

C

$$X_0 + X_1 + X_2 + ....... +X_n$$

D

$$X_0X_1X_3......X_{n - 1} +X_2 + X_3 + X_5......X_{n - 1} + ........ + X_{n - 2} + X_{n - 1} + X_{n}$$