Final answer:
During RecA-mediated presynapsis in strand exchange, single-strand invasion (option C) occurs, where a nucleoprotein filament formed by RecA binds to single-stranded DNA and invades the homologous double-stranded DNA to initiate repair.
Step-by-step explanation:
During the RecA-mediated presynapsis stage in strand exchange, single-strand invasion occurs. This is the process where RecA protein binds to single-stranded DNA, forming a nucleoprotein filament. This filament then conducts a search for a homologous sequence of DNA on the opposite strand. Upon locating a match, the filament invades the double-stranded DNA, pairing its 3' end with the homologous sequence and displacing the original complementary strand. This displacement forms a D-loop, enabling the exchange and repair process to continue. This step is critical for the homologous recombination repair mechanism, particularly for repairing DNA damage such as single-stranded breaks.