The presence of a GC-rich sequence followed by a series of A residues in the DNA template strand results in the termination of transcription because:

GC-rich sequence in the RNA transcript forms a stem-loop structure which destabilizes the RNA-DNA hybrid and causes the RNA polymerase to pause.

The series of U residues in the RNA transcript is weakly paired with the A residues in the DNA template strand.

The weak interactions between the U residues in the RNA transcript and A residues in the DNA template strand is unable to hold the RNA-DNA hybrid together, leading to transcription termination.

All of the above.