If you ___ the bill now, we'll return 5 percent to your credit card.

Grammar Rule:

The sentence uses the first conditional structure, which describes a real or likely situation in the future and its probable result. The structure is: If + present simple, will + base form of the verb.

Solution:

1. The sentence describes a condition (paying the bill now) and its consequence (returning 5 percent).
2. This indicates the first conditional tense is appropriate.
3. The correct form for the 'if' clause in the first conditional is the present simple.
4. Therefore, 'pay' is the correct verb to fill the blank.

Answer: pay