The polio vaccine currently administered in the u.s., known as ipv, is an example of a(n) vaccine.
(a) recombinant attenuated.
(b) whole inactivated.
(c) subunit.
(d) toxoid.

(C) subunit.

Explanation:

A vaccine is a substance derived from, or chemically similar to, a disease-causing infectious agent. Regardless of their nature, these substances are presented to the cells of the immune system, which learn to recognize and fight them during infection.

The polio vaccine is a subunit type vaccine. This type of vaccine contains fractions or subunits of the infectious agent, which in this case is the virus that causes polio, selected because of its ability to initiate a specific immune response.

Option D. "Toxoid"

Explanation:

Inactivated polio vaccine (IPV) is defined as the polio vaccines which are used to prevent poliomyelitis.

IPV is produced from wild-type poliovirus strains and then they are allow to  inactivated (killed) with formalin.

Toxoid vaccines is an example of IPV, which is made by inactivating the toxin that is responsible for disease symptoms  and allowed an immune response against the toxin without any harm to the body. Toxoid vaccines are available for several immunizations including the tetanus and diphtheria immunizations.

Hence, the correct option is D.