Let's use process of elimination to answer this question!
D is wrong, and we know this because of the names of DNA and RNA. The D in DNA stands for deoxyribose, and the R in RNA stands for ribose. These are the types of sugars in each nucleic acid. Therefore, they don't both contain a ribose sugar.
C is wrong, and this is because RNA does not contain thymine, but uracil instead.
B is wrong, and we know this because we know that RNA is composed of only one strand, and therefore it is not composed of multiple stands (plural).
Therefore, A must be right. We can confirm this, because we know that polymers are large molecules consisting of repeated subunits. DNA and RNA are both polymers consisting of nucleotides.
