The correct answer is A. Elisa test.
An ELISA test is a blood test that can detect antibodies to HIV. Antibodies are proteins that the body produces in response to an infection. If a person has HIV, their blood will contain antibodies to the virus. The ELISA test is a very sensitive test, and it can detect HIV antibodies even when the amount of virus in the blood is very small.
A hybridization test is a type of test that can be used to detect the presence of specific DNA sequences. This type of test is often used to diagnose genetic diseases. However, it is not used to detect HIV infection.
Gram staining is a type of test that is used to classify bacteria. This type of test is not used to detect HIV infection.
Therefore, the correct answer is A. Elisa test.