a. Yes, possible, and very natural. "It has rained for two hours" implies that it's still raining. "It rained for two hours" implies that the rain has stopped. b. It would be better to say "It rained for two hours". Please understand that the difference in meaning is slight. Even native speakers confuse tenses sometimes.

a) It makes sense, but sounds a little awkward. If I looked out the window and saw it was raining, I would say "It has been raining for two hours now." If it is no longer raining, I could say "It rained for two hours" If I were telling somebody about it later, I could say "It had been raining for two hours [when I decided to leave]." b) I might say "The ground is wet; it was raining for two hours." It has been raining means it is still raining.

a. "It has rained for 2 hours" - means that it is still raining. You may say this to indicate a result or fact that has continued to the present ("so far") or perhaps you expect a change in the weather. b. "It has been raining for 2 hours" means that it is still raining now. It started two hours ago and is continuing in the present. The continuous tense emphasises that this is an activity. If you want to emphasise that an activity started in the past and is continuing in the present, then use the present perfect continuous.

a) If it is, in which context? It could only mean that it isn't raining anymore? No. If you use the Present Perfect, it has rained for two hours, you are including the present time in your statement, and one could imply that it is still raining even though it started two hours ago. If it is finished, then you would simply use the past tense: it rained for two hours. Finished. b) Also, is it ok to say "It isn't raining now, but the ground is wet. It has been raining for two hours"? No again. If it isn't raining now, but the ground is wet, that means that it rained some time slightly before then. It rained, or it had rained or it was raining earlier.