Tactics that (can) rise from early game positions.
Tactics
Why could you only study one type of tactic? I own over a thousand chess books and have yet to see one devoted to only one tactic. I'd recommend using the Tactics Trainer on this site. You get a rating so you are working with tactics in your range. As you improve you get stronger problems. But just don't grind them out, play over and understand each one before moving on. If you are stumped, you can ask as each tactic has its own forum.
Best
Which is the most important basic tactic to study, what i mean by that is if you could only do one type of tactic puzzle, which would it be?