How are public and private keys mathematically related in ElGamal?

Select one:

a$.$ They are exponentiated with one another to obtain the public key exponent.

b$.$ The private key is the discrete logarithm of the public key in the chosen finite field.

c$.$ The private key is obtained by inverting the public key modulo the prime number.

d$.$ They are simply additive inverses of each other modulo a prime number.