I'm sure I haven't caught all the errors in each answer choice, but I'll take a stab at explaining my choice.
Quote:
Proceeding without a definite plan for upcoming labor negotiations, like the firm had agreed to last year, it would surely have proven to be a disaster in the face of the skilled and resolute opposition involved this time.
At first glance, this is a run-on sentence due to the "it." I'll rephrase it in a clearer way and remove the extra information.
"Last year, the firm agreed to follow a definite plan. Proceeding without that plan would have proven to be a disaster."
The conditional "would (surely) have" in the original sentence tells me that the firm followed a plan, and the result did not end in disaster.
Quote:
A. Proceeding without a definite plan for upcoming labor negotiations, like the firm had agreed to last year, it would surely have proven to be a disaster
Incorrect. Run-on sentence. "Had agreed" might be the wrong tense.
Quote:
B. Proceeding without a definite plan for upcoming labor negotiations, as the firm agreed last year to do, would surely have proven to be a disaster
"As the firm agreed last year to do" sounds a bit strange, and at first I got tripped up because I thought "do" was referring to "plan," and one does not "do" a plan. So what did the firm agree to do? To proceed. Looks fine, but I'll check the other choices.
Quote:
C. Going ahead without the presence of a definite plan for upcoming labor negotiations, like the firm had agreed last year to do, would surely have proven disastrous
Incorrect. "Presence of a definite plan" changes the meaning of the sentence. Also, "like" should be swapped for "as."
Quote:
D. To proceed without the presence of a definite plan for upcoming labor negotiations, as the firm had agreed to last year, would surely have proven disastrous
Incorrect. "Presence of a definite plan" changes the meaning of the sentence. Also, when rephrased, "To proceed as it agreed to" sounds wrong - it should be "To proceed as it agreed to do."
Quote:
E. Going ahead without their having a definite plan for upcoming labor negotiations, as they agreed to last year, it would surely have proven to be a disaster for the firm
Incorrect. Another run-on sentence. "Their" and "they" are not the correct pronouns for "firm." "Without their having" is an unnecessary change that is awkward and verbose. "Proceeding" is changed to the more informal and more unclear "going ahead." My opinion is that this is the worst answer choice if we're comparing the number of errors in each choice.
By process of elimination, the answer is B.