Hi I wonder if anyone can help me?

I have created a projects tab (which details events for our company) when I added an activity it gave me the option in the related to drop down menu to link it to an event.
I seem to have deleted this option and when i look in the drop down menu it only relates to events, accounts, contacts and tasks rather than the individual events.

I hope this makes sense and would be very grateful if anyone new how to get this back.

Many thanks