If you are sure that you really can't explain it, doubt the answer. The first answers to this question are a and b, because contrary to the meaning of the sentence, you can't train before your knee recovers, nor before your knee recovers. Note that until before should be followed by an affirmative sentence. Since it is a negative sentence, it completely violates the meaning of the sentence. Although before and until are different in details, the meaning of the sentence is similar. Since both of them violate the meaning of the sentence, there is no need to discuss their differences in details. Choose between c and d. There is no such usage in English. Consideration is equivalent to thinking, which leads to the adverbial of reason. Considering that your knee hasn't recovered yet, you can't receive training.