The answer is d. Why is "certain spicy foods" not a subject phrase/singular?
The reason that certain spicy foods, such as the Habanero pepper, makes some people sweat is because they contain a chemical that stimulates the same nerve endings in the mouth as does a rise in temperature; this stimulation sometimes results in the activation of certain biological cooling mechanisms, one of which is perspiration.
a. The reason that certain spicy foods, such as the Habanero pepper, makes some people sweat is because they contain a chemical that stimulates the same nerve endings in the mouth as does a rise in temperature
b. The reason that certain spicy foods, such as the Habanero pepper, makes some people sweat is that these foods contain a chemical that stimulates the same nerves in the mouth like a rise in temperature
c. Certain spicy foods, such as the Habanero pepper, make some people sweat because they contain a chemical that stimulates the same nerves in the mouth like a rise in temperature does
d. The reason that certain spicy foods, such as the Habanero pepper, make some people sweat is that these foods contain a chemical that stimulates the same nerves in the mouth and on the tongue as does a rise in temperature
e. The reason that certain spicy foods, such as the Habanero pepper, make some people sweat is because they contain a chemical that stimulates the same nerves in the mouth as does a rise in temperature