DNA is the genetic material that is present in the cells of our bodies, and as is unique for each of us, can be used by the police to identify someone.
As DNA is found in the cells, we can discard the options that do not involve cells, such as fingerprints and tears. Fingernails and dandruff (scalp skin) have DNA, but as the place where we are looking for DNA is a ski mask, it's more likely to find dandruff than fingernails.
This means A, dandruff would be the correct answer.