Answer: will rain
Explanation:
There are a lot of clouds! It will rain soon.
A verb will be needed to convey that an action is to be done which is that there will be rain. The verb will also have to be transitive in order for it to transfer the action to the word, ''soon''.
Will rain is the proper phrase to put there as it is a transitive verb that affirms that an action will take place which in this case is that there will be rain.
