Low gear, sitting, and more pedaling vs higher gear, standing, less pedaling.

Which is more energy efficient?

This.

Definitely the sit and spin.

I agree with the others but standing is more fun - to me anyway.

Sit and spin - but its so much fun when I can "dance in the pedals" like a TdF mountain top finish.

Spinning also produces less fatigue.

Consider kicking off a 418 mile supported tour with a 6000' climb including a 20 mile, 4700' stretch averaging 4.5% and reaching 6%.

Ending the day with extra Cliff bars isn't going to win you anything, but having the legs left to finish the day quickly will get you a prime camping spot with no line for a hot shower and still feeling good tomorrow will make the following 330 miles and 24,000 feet much more pleasant.

Seated high cadence spinning punctuated by heart pumping bouts of pedaling out of the saddle at high cadence. Standing and pedaling a "too high" gear and you diminish efficiency.

Sitting and spinning uses less energy than standing up and pushing larger gear, That does not mean you should not stand up at all. On a long climb, it is a not a bad idea every so often to shift up a gear or two and stand up foir a little bit, then sit down, shift back down, and resume your regular pace. This changes the muscles you are using a little bit and lets the one used when sitting to rest a bit.

This all assumes it is a "rhythm" climb, one on which you can fairly easily keep up a reasonable seated cadence for an extended period. On a steeper "power" climb, you do what ya gotta do to get - or what your legs and lungs can tolerate.

Sitting and spinning is more efficient. Often I ride with a group that's faster than I am. That's when the extra energy required by standing is especially noticeable. If I'm already struggling to hold the pace, the extra energy consumed by standing will just slay me.

If you meant your question literally, then yes, as the others have stated, sitting and spinning is the answer. But besides "getting to the top," there can be as many preferred outcomes to climbing a hill as there are climbers. Some want the most energy efficiency, some want to get there the fastest, some want some combination of the two, driven by what they perceive their energy needs and time requirements are for the rest of the ride.