Explanation
Correct answer: A.
Choice A is the best answer. The convention being tested is the use of finite and nonfinite verb forms within a sentence. The nonfinite to-infinitive “to forge” is correctly used to form a nonfinite (infinitive) clause that explains why the chemists re-created and reprogrammed the DNA-cleaving bacteria.
Why the other choices are wrong
Choice B
Choice B is incorrect. Without a comma separating the main clause (“chemists...bacteria”) from the participle “forging,” this choice illogically suggests that the bacteria are forging a tool, which doesn’t make sense.
Choice C
Choice C is incorrect. Without a coordinating conjunction such as “and” placed before it, the finite past tense verb “forged” can’t be used in this way to describe the chemists’ actions.
Choice D
Choice D is incorrect. If read as a finite verb, the present progressive verb “forging” isn’t consistent with the past tense verbs used in this sentence to describe the actions of the chemists. If read as a nonfinite verb, the participle “forging” can’t be used in this way because there is no following main clause for it to modify.