Section 301 of the 1988 Omnibus Trade and Competitiveness

If it makes anyone feel better, I'm 100% certain I would have gone for D as well. I have looked at different grammar sources and none say that "conclude to" is incorrect, and many times I found that "conclude by" is correct. The main point is the context of the usage.

Example #1:
After reviewing the scouting report, our coach will conclude to use trick plays.

Example #2:
According to the state Constitution, the Oklahoma legislature must conclude by the last Friday in May.

Example #1 uses "conclude" like making a decision.

Example #2 uses "conclude" in the manner of "wrap up" or "finish", and this is the same usage as in our question.

Example #1 is incorrect. When used in the context of thoughts or decisions, "conlucde to" is not correct. A person will "conclude that" he should use trick plays.

Example #2 is correct because the manner of usage is to wrap up. This requires some event. Like, the game will conclude when an event takes place, or by some event taking place. "The innining will conclude when the defensive team has made 3 outs."

In the example sentence of the question, the negotiations will conclude when an event takes place. The event is when the U.S. Government is satisfied. So we need "conclude by".

So here, B is correct because it uses the phrase "conclude by" in the same manner as "wrap up". I hope this helps.

For full disclosure, Mrs. Jallenmorris helped out a lot with answering this question.