Hello!
Let's go through the given choices to find which word is a synonym for knocking.
A. Rapping means to strike something, specifically hard, loudly, to attract attention.
B. Ticking means to make sharp, short movements like a clock does.
C. Thundering means to make a loud, deep noise.
D. Popping means to make an explosive sound.
Therefore, the answer is A, rapping because knocking is striking a surface to get attention, which is similar to rapping.