________ such a terrible earthquake hit the area, no one can tell how devastating the aftermath will be

Question

________ such a terrible earthquake hit the area, no one can tell how devastating the aftermath will be.

1. That
2. Could
3. When
4. Should
5. Because

The answer is 'should'. 'When' is impossibe because if you use 'when' you should change 'hit' to 'hits'. So I wonder 'if' is possible?

Q1) "If such a terrible earthquake hit the area,~" is this sentence possible? (specially in grammar)

If possible

If such a terrible earthquake should hit the area, ~ (hit = verb root)
If such a terrible earthquake hit the area,~ (hit = past verb)

Q2) What is difference between two sentences?

Answer 1

The fact that the main clause is in the future tense (will) would lead me to think that the hypothetical premise should be in the present tense:

If such a terrible earthquake hits the area, no one can tell how devastating the aftermath will be.

But that of course is not possible, since we neither have "if" among the answers nor "hits" in the sentence.

Certainly "should" makes more sense, even though that would imply, in my mind, that a second coinditional sentence should be formed, that is:

Should such a terrible earthquake hit the area, no one can tell how devastating the aftermath would be.

As for your two examples with "if", they are both acceptable, and equal in meaning.