Penicillium (the source of the antibiotic penicillin) is a mold that secretes a toxin that kills bacteria and other organisms. Penicillium, however, is not harmed by this toxin. This is an example of: a. predator-prey.
b. commensalism.
c. mutualism.
d. amensalism.

This is an example of amensalism (Option d).

Explanation:

The interaction between penicillium and bacteria is a typical example of amensalism.

Amensalism refers to a biological relationship established between two organisms, in which one of them avoids the growth, development, or survival of the other one. In this interaction, one of them is strongly affected by the other one, which does not seem to be affected at all. Antibiosis is referred to as amensalism interaction.

Antibiosis is the interaction in which one of the species secretes a chemical that kills the other species organism or inhibits its actions by causing severe damage. The organism that releases the toxic substance is called the inhibitor. For the inhibitor species, this relationship is neutral because it does not seem to be beneficiated or affected.    

Another classic example of amensalism is those plants that secrete chemicals to damage or kill other plants that live near them. In this way, the secretory plants may live and grow with no competition.