Which of the following lacks DNA?
There are two answers if the question refers to genomic DNA (neither enucleated ova nor mature RBCs have genomic DNA).  However, since the question doesn't specify genomic DNA, we can exclude enucleated ova, which would still have mitochondria and therefore mitochondrial genomic DNA.  Mature RBCS (at least in most mammals) do not have mitochondria, so the safest answer is probably b) Mature RBC.