The correct answer is C. Besides.

1. Context of the Sentence:
- The sentence explains a reason for not wanting to go to the party, and "besides" is used to add another point or reason to the already stated one.

2. Word Meanings:
- Beside: This means "next to" and does not fit the context of adding a reason.
- Aside: This means "to one side" or "in a different context" and does not fit the sentence.
- Besides: This means "in addition to" or "apart from" and is used to add another reason.
- Apart: This means "separate" and does not fit the context of adding reasons.

3. Usage in the Sentence:
- "Besides" is used correctly to introduce an additional reason for not going to the party, making the sentence: "I don't want to go to the party besides I was not invited."