Mike wants to watch TV ______ tonight ?

In the sentence "Mike wants to watch TV ______ tonight," the blank can best be filled by the article "a," which implies that Mike is looking to watch a particular program or show but does not specify which one. In this context, “a” serves as an indefinite article, indicating a non-specific instance of something.

Using an indefinite article such as "a" is appropriate when referring to something for the first time or when it is one of many possibilities. For example, saying "Mike wants to watch a movie tonight" suggests he is open to different options and hasn't chosen a specific title yet.

The other options do not fit well in this context:

- "the" would suggest a specific TV show or program that both the speaker and the listener are aware of, narrowing down the choice.
- "an" is used before words starting with a vowel sound, which does not apply here since "TV" starts with a consonant sound.
- Leaving the blank empty ("_") would leave the sentence grammatically incomplete.

Thus, "a" is the most suitable option, as it allows for the flexibility of choice and fits the sentence structure appropriately.