Which hypothesis is testable?

Plants that are grown by using brand A fertilizer taste best.

Plants that have flowers look prettier than plants that do not have flowers.

Plants will eventually die off if global warming trends continue.

Plants make more sugar on sunny days than on cloudy days

The correct answer is Plants make more sugar on sunny days than on cloudy days. That's because the first two involve personal taste since flavor of something and something being pretty is subjective so you can't test it. The global warming part is untestable because we can't create global warming just to test whether the plants will survive.