Which one of the following pairs of diseases are viral as well as transmitted by mosquitoes? 

Yellow fever and dengue

The answer is (d). Yellow fever and dengue are both viral diseases that are transmitted by mosquitoes.

Elephantiasis is not a viral disease, and ringworm is not transmitted by mosquitoes. Malaria is a parasitic disease that is transmitted by mosquitoes, but it is not a virus.

Here is a table of the diseases mentioned in the answer choices, along with their transmission method and whether they are viral: