Viral hemagglutination assays only work with certain types of viruses because
the virus must be able to cross-link red blood cells directly.
the virus must be able to lyse red blood cells.
the virus must not be able to lyse red blood cells.
other viruses are too dangerous to work with in a clinical lab setting.

a. the virus must be able to cross-link red blood cells directly.

Explanation:

Viral hemagglutination assay is a method which is used for the quantification of the relative concentration of the viruses.

The principle on which the hemagglutination assay works is as follows:

Thus, this method allows knowing the relative concentration of the virus. 

Due to its basic principle of cross-linking this method works well for only a few viruses such as influenza virus. 

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explanation:

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genetic variation can be caused by mutation (which can create entirely new alleles in a population), random mating, random fertilization, and recombination between homologous chromosomes during meiosis (which reshuffles alleles within an organism's offspring).