What is required to activate a cytotoxic t cell

Question

Problem:

Question: What is required to activate a Cytotoxic T cell?

1. A known antigen bound to MHC-I
2. A Helper T cell bound to an antigen and MHC-II and macrophage
3. Any antigen on an MHC-II, whether new to the body or not
4. An antigen that has been encountered previously

Please select correct answer and also provide explaination.