Engineered herpes viruses are being used to treat cancer because the virus a. was engineered as an oncovirus, only infecting tumor cells. b. destroys the blood vessels feeding the tumor. c. prevents apoptosis. d. marks the location of tumors for surgical removal

a. was engineered as an oncovirus, only infecting tumor cells

Explanation:

Now-a-days a lot of onco-viruses are in development to fight cancer. These viruses are engineered specifically to target tumor cells. They reside in tumor cells and can be used as marker where they elicit very potent immune responses by antigen representations necessary for activation and recognition of tumors by immune system itself.

a. was engineered as an oncovirus, only infecting tumor cells.

Explanation:

Oncoviruses such as the engineered herpes virus is used a tool for killing cancer/tumor cells. When a virus infects a cancer cell, it makes several copies of itself within the cancer cells until the cell burst. The oncoviruses only target tumor cells, it does not affect healthy normal cells.

In addition to infecting tumor cells, it also triggers the immune system against the cancers.  

Coronavirus disease (COVID-19) technical guidance: Laboratory testing for ... to laboratories and stakeholders involved in laboratory testing of COVID-19 virus. ... Diagnostic testing for COVID-19 is critical to tracking the virus, understanding ... 2. WHO interim guidance for laboratory biosafety related to COVID-19 virus.

Explanation: