The answer is d. deletion.
If the DNA sequence is TTCACG, it is expected to be transcribed into AAGUGC.
But, it was transcribed into AAGGC. If we count nucleobases, a transcribed sequence has 5 nucleobases, while DNA sequence has 6 nucleobases. These means that deletion happened.
If insertion occurred, there will be 7 nucleobases in transcribed sequence. In substitution or translocation, the transcribed sequence will be the same size, but with different sequence than the DNA sequence.
