Look at all those black clouds! It……..today.

Sentence is in present continuous tense with active voice.
Rule :
Subject + is/am/are + V-ing + other agents.
Look at all those black clouds! It is going to rain today. The correct answer is Option A: is going to rain.

Let's understand why:

We use 'be going to' (like 'is going to') when we make a prediction about the future based on evidence we can see right now.
In this sentence, "Look at all those black clouds!" is the clear evidence.
Black clouds are a strong sign that it will rain soon.
So, "It is going to rain today" means we predict rain because we see the black clouds now.

Now let's look at the other options:

Option B: is raining
This means it is raining at this very moment. But the sentence is about looking at clouds *before* it starts to rain, not during.

Option C: will be rain
This option is grammatically incorrect. We say "it will rain" or "it will be raining", but not "it will be rain".

Option D: rains
This form (simple present tense) is used for things that happen regularly or habitually, like "It rains every winter." It doesn't fit a specific prediction based on current evidence for 'today'.

Therefore, Option A is the best choice because it correctly uses 'is going to' for a prediction based on strong present evidence (the black clouds).