You use a comma in a sentence primarily for the following reasons:
- To show a pause in a sentence or separate items in a list.
- To indicate an abrupt break in thought or structure.
- To denote an omission (for example, in a sentence where a word is left out).
Commas are not used to mark the end of a sentence; that function is served by a period.
So, the best answer from your options would be: to show a pause in a sentence or separate items in a list and to indicate an abrupt break in thought or structure.