Which activity occurs during the s stage of the cell cycle

The right answer is Replication of DNA.

The phase of synthesis of DNA or phase S is the phase where the cell replicates (duplicates) its DNA in two identical copies.

The duration of this phase is variable, from a few minutes for embryonic cells to several hours for most somatic cells. For these, indeed, during this phase, the cell continues to actively transcribe the genes of the proteins necessary for their survival and maintenance of specialized functions.

During phase S, the cell also doubles the centrosome required for chromosome migration. Both centrosomes migrate around the nucleus and position themselves diametrically opposite.

the answer to your question is:

c. complementary base-pairing