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:

There are sialic acid receptors present on the surface of the red blood cells which will bind to the haemagglutinin glycoprotein present  on the surface of the viral particle. Due to the binding of the sialic acid receptors with haemagglutinin particle there will be cross-linking of the red blood cells by the viral particles and a lattice formation will take place. The lattice will remain suspended in the solution however if the concentration of the virus particles will be less, the lattice will settle down in the well. 

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. 

The answer is joseph is observing the colour of the reaction mixture to see whether proteins are present in the given solution.