Lets first do process of elimination. The question is asking ''Yesterday I blank you started work'' so lets see which Spanish verb completes the sentence. First cross out A) because that would be ''we knew'' and that wouldn't make sense in the sentence. Cross out C) because that means ''he knew'' and that doesn't make sense in the sentence. Cross out D) because that means ''they knew'' and that doesn't make sense in the sentence. In conclusion the answer would be B) ''I knew'' because it completes the sentence. Ayer yo supe que empezaste a trabajar is correct.