When using public key cryptography to send data to another host, when is the receiving hosts public key used?

Question $1$ options:

a$)$

It is used when the sender adds a digital signature.

b$)$

It is used when the receiver decrypts the data.

c$)$

It is used with the sender encrypts the data.

d$)$

It is used when the sender encrypts the data and when the receiver decrypts the data.