To examine the mutations in the two DNA strands:
Original strand: AGC GTA CCC TAC
New strand: AGC GTA ACC TAC
Comparing these two strands, we can see that the original sequence "CCC" has changed to "ACC". This indicates that one base pair (C) in the original sequence has been substituted with another base pair (A) while the others remain the same.
Given this, the kind of mutation that occurred is a substitution.
So the correct answer is: substitution