The correct answer is A. Thymine.
Thymine is a pyrimidine base found in DNA but not in RNA. It pairs with adenine via hydrogen bonds. Uracil is a pyrimidine base found in RNA but not in DNA. It pairs with adenine via hydrogen bonds. Ribose is a pentose sugar found in RNA and DNA. Phosphate is a group of three oxygen atoms and one phosphorus atom found in RNA and DNA.
DNA is a double helix structure made up of two strands of nucleotides. Each strand is made up of a sugar-phosphate backbone with nitrogenous bases attached. The nitrogenous bases on one strand pair with the nitrogenous bases on the other strand, with adenine always pairing with thymine and guanine always pairing with cytosine. This complementary base pairing allows the two strands of DNA to be held together.
RNA is a single-stranded molecule made up of a sugar-phosphate backbone with nitrogenous bases attached. The nitrogenous bases in RNA are adenine, guanine, cytosine, and uracil. Uracil is found in RNA instead of thymine. This is because RNA is involved in protein synthesis, and uracil can pair with adenine just like thymine can.
The phosphate group is found in both DNA and RNA. It is attached to the sugar-phosphate backbone and helps to hold the molecule together.