Viruses are unique among infectious agents because they are?
a. microscopicb. deadly
c. harmless
d. not living

It's answer d, but for info :

The question of wether or not a virus is alive is not resolved, some scientists say yes, some say no.

The fact is, viruses need another organism's enzymatic machinery to survive : alone, they cannot multiply or grow, produce or use energy, functions that are common to everything alive.

On the other hand, they do possess nucleic acids and proteins like living beings!

Bis the correct answer.