After careful consideration I have decided not to tile over my oak
floor and instead rip it out down to the subfloor, and would like to
install plywood and backer board for a new tile install. I have 3/4"
subfloor and can only go an additional 3/4" more due to height
problems. So here are my options to put over the 3/4" subfloor.

a) 3/4" plywood
b) 1/2" plywood plus 1/4" backer
c) 1/4" plywood plus 1/2" backer

Either choice would give me a combined floor of 1 1/2". So which would
be best?