The correct answer is in bold. Methotrexate is an inhibitor of dihydrofolate reductase. Pyrimidine synthesis doesn't involve the one-carbon pool. The methyl group that appears in dTMP is synthesized during the transfer of the methylene group from the one-carbon pool. This conversion involves an oxidation and an amination but no additional carbons.
Purine synthesis requires two components of the one-carbon pool.
In the absence of this enzyme, dietary folate could not be reduced to the tetrahydro form necessary for carbon carriage.