Today, many biologists and anthropologists agree that the most intense selection pressures our species faces come from

The major threat to humans and the most prominent selection pressure are disease organisms that target the immune system.
These organisms impair the body's ability to fight off invasion from pathogens, and there is no "back up immune system" to protect the immune system from attack. An example of a pathogen that attacks the human immune system is the Human Immunodeficiency Virus (HIV).

Ithink the answer is d.