Viruses can enter animal cells by
a. poking holes in their membranes and entering
b. triggering an active transport mechanism
c. endocytosis
d. exocytosis

Endocytosis

Explanation:

The viruses have some structures such as an envelope, spikes, and capsids. Certain viruses lack an envelope called naked viruses. The viruses enter into the host cell in two ways. In one method the virus envelops fuse with the cell membrane of the host. This is due to some viral proteins present in the viral cells. Then the virus capsid enters the cytoplasm and releases its viral DNA. The capsid is digested by the cytoplasm of the host cell.

Another way of virus entry into the cell is endocytosis. Here the enveloped viruses adsorbed into the plasma membrane of the host by spikes. The spikes of the virus are proteins that attach to the cell membrane.  

Then the viruses trapped in the cellular vesicle and taken into the cytoplasm. Then the envelope disintegrates in the vesicle and nucleocapsid is released to the cell. Then the viral DNA replicates in the host cells. The naked virus also enters the host cell by endocytosis.