What is the one-way function mathematics that El Gamal is based on?

What is the one-way function mathematics that El Gamal is based on?

A.
Calculating logarithms in groups and an infinite field

B.
Calculating logarithms in a finite field

C.
Calculating logarithms on curves

D.
Factoring large numbers into their original prime numbers

Explanation:
The hard direction is based on a hard mathematical problem. RSAs hard
mathematical problem is factoring large numbers into their original prime numbers.
Diffie-Hellman, El Gamal, ECC are based on the difficulty of calculating logarithms
in a finite field.