Step 1: Common bases in DNA and RNA.
Both DNA and RNA contain the purine bases adenine (A) and guanine (G), and the pyrimidine cytosine (C). These three bases are found in both types of nucleic acid.
Step 2: The distinguishing base.
The fourth base differs: DNA contains thymine (T), while RNA contains uracil (U) in place of thymine.
Step 3: Structural basis.
Thymine is 5-methyluracil; it has a methyl group at the 5-position that uracil lacks. This methyl group is specific to DNA.
Step 4: Answer identification.
Among the options, guanine (A), cytosine (B), and adenine (D) are present in both nucleic acids. Only thymine (C) is exclusively in DNA.
\[ \boxed{(C)} \]