The correct answer is: since.
The word “since” is used to indicate a point in time in the past from which something continues to happen or be the case. In this sentence, the speaker is saying that they have been working hard from a point in time in the past, which is last year.
The other options are not correct because they do not indicate a point in time in the past. The word “in” is used to indicate a period of time, but it does not necessarily indicate a specific point in time. The word “from” is used to indicate a starting point, but it does not necessarily indicate a specific point in time. The word “for” is used to indicate a duration of time, but it does not necessarily indicate a specific point in time.
Here are some examples of how the word “since” can be used in a sentence:
- I have been working hard since last year.
- I have been living in this house since I was born.
- I have been a fan of this band since they first started out.