The correct answer is A. since.
The word “since” is used to indicate the beginning of a period of time. In this case, the speaker is saying that it has been raining since morning. This means that the rain started at some point in the morning and has continued to fall ever since.
The other options are not correct because they do not indicate the beginning of a period of time. The word “for” is used to indicate a duration of time. In this case, the speaker could say “It has been raining for two hours.” The word “till” is used to indicate a point in time up to which something happens. In this case, the speaker could say “It will rain till noon.” The word “by” is used to indicate a point in time by which something happens. In this case, the speaker could say “The rain will stop by nightfall.”
Here are some examples of how the word “since” can be used in other sentences:
- I have been working here since 2010.
- We have been friends since we were kids.
- I have not seen her since last week.