Flange opening minimums

Rental house build in 1977 - Had half bath in basement - Vanity and shower - Recently tore out vanity and shower - Wanted to replace vanity along with adding a toilet - Where shower stood has 2 inch high 24X24 concrete riser platform that has a 4 inch hole but only has a 2 1/2 inch sewer pipe - This pipe is cut flat with the riser - Can only see approx 12 inches below the flooring and it looks like the pipe remains the same size -

What options do I have to put a flange down to install a toilet?

that 2 inch pipe is for the shower drain. for a toilet you need at least 3 inch pipe. you would probably have to break up the floor and find the sewer and cut a wye into it for the toilet or break up the floor and let a plumber do it for you.

I must assume by the nature of your question that you have no experience with toilet drains. This is the time a wise DIYer gets professional help because a toilet that is not connected correctly will cause constant grief. Your tenants will be calling you in the middle of the night and on week-ends with a clogged toilet. I advise getting a plumber to evaluate what has to be done, and what you can do, such as breaking out concrete, and other grunt work. Then he can come in and make short work of the drain connection.