The correct answer is D. 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 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 as accurate. The word “for” is used to indicate a period of time during which something happens or is the case. In this case, the speaker is not saying that it has been raining for a specific period of time. The word “from” is used to indicate the starting point of something. In this case, the speaker is not saying that the rain started at a specific time. The word “till” is used to indicate the ending point of something. In this case, the speaker is not saying that the rain will stop at a specific time.