This is similar to your other question. Think about it this way: you're asked to find out at what time would the machine fill the other?

In order to answer the question, you would need to know some combination of the following:

1. What rate does the machine work at?
2. When did the machine start working?
3. When did the machine stop working?
4. How much of the job did the machine complete?

If you glance over the answer choices, they provide you with 2, 3, and 4 from the above. Knowing what time the machine started and ended and how much of the job it completed in that time allows you to compute the rate at which the machine works at.

If this is confusing, think differently - if the machine started at 930AM, does this allow you to answer the question? No. By itself, the start time means nothing, it could have taken 1 minute, or 1000 hours. You have no clue. So A is out. Likewise, if the machine ended at 1030AM, does it answer the question? Nope - same as above. Put these together now - if the machine started at 930AM and finished half the job at 1030, you can deduce that the machine will complete the full job at 1130 (half job in 1 hour so full job in 2 hours); hence, C is the answer.