definitely SNA - Orange County, CA
Hi - The closest airport to Cal State Fullerton is definitely Orange County Airport (SNA). It's probably a 25 minute drive in no traffic. From SNA, take 55 north to 91 west to 57 north and exit State College.
The second closest airport to CS Fullerton is Ontario, and Long Beach is perhaps a bit farther.
LAX is fourth closest - a 50 minute drive in no traffic, but 1.5 hours if there's traffic.