Crucial in the decision to sell the company, a decision which made some 300 employees redundant, was the numerous licensing issues that plagued the system that was designated to be the company's flagship product.

A. a decision which made some 300 employees redundant, was the numerous licensing issues that
B. a decision that made some 300 employees redundant, has been numerous licensing issues which
C. was the numerous licensing issues that led to a decision which made some 300 employees redundant and that has
D. a decision that made around 300 workers unemployed, were the numerous licensing issues that
E. a decision that made some 300 employees redundant, was the numerous licensing issues which

The correct answer is D.

This answer choice corrects the original Subject Verb Agreement mistake by changing the singular verb was to the plural subject The numerous licensing issues.

Answer A - Incorrect.

This answer choice is grammatically incorrect. The plural subject the numerous licensing issues does not agree with the singular verb was.

It is much easier to identify the subject in this sentence, if we change the word order so that the subject comes before the verb, as in most sentences:

The numerous licensing issues that plagued the system that was designated to be the company's flagship product was crucial in the decision to sell the company, a decision that made some 300 employees redundant.

We can now see the Subject Verb Agreement mistake more clearly.

Answer choice B repeats the original Subject Verb Agreement mistake: the plural subject the numerous licensing issues does not agree with the singular verb has been.

Answer C changes the sentence's word order, yet it repeats the original Subject Verb Agreement mistake. The plural subject the numerous licensing issues does not agree with the singular verb was. So it is also incorrect.

Answer choice E repeats the mistake which we see in answer C.