Answer:
a) ,which had been recorded earlier, was put online.
Step-by-step explanation:
a) is the correct answer. Comma sings in this sentence are put properly in order to make an apposition. This is grammatical construction where elements are put side by side, one being separated between commas and serving a purpose to explain the first element. In this case, the apposition saying the lecture had been recorded earlier is explaining the elements of the lecture.
b) is not the right answer as commas put this way are not correct.
c) is not correct. Putting word which between commas doesn’t make grammatical sense.
d) is not correct. The presence of the second comma without the first one is not grammatically proper.