I'm no expert, but I study biology, hope the following helps! :) If you think my answer is wrong, please do give feedback, we are all here to learn :)
A and C are similar and vague, and should be striked off.
D is incorrect because the question specifies that Actinomyocin-D blocks DNA replication. The blockage of DNA replication does not necessarily mean death for cells.
B is a possibility, since DNA replication is similar in both Tumour cells and normal cells, and so the drug prevents all cells from dividing, even the healthy ones.