Explanation:

- Rahul thinks that this is quite the cheap restaurant.

- The phrase "quite the cheap" is not typically used in English. It doesn't fit the common structure.

- Option 1: so the cheap

- This option doesn't correct the sentence. It remains awkward and incorrect.

- Option 2: No substitution

- Keeping the original sentence doesn't sound natural or grammatically correct.

- Option 3: quite a cheap

- This option makes the sentence sound natural and correct. It aligns with usual expressions like "quite a nice place."

- Option 4: so a cheap

- While slightly better, it's not the correct choice for this sentence.

The correct option is: Option 3: quite a cheap