Which hypothesis is testable?
a. plants that are grown by using brand a fertilizer taste best.
b. plants that have flowers look prettier than plants that do not have flowers.
c. plants will eventually die off if global warming trends continue.
d. plants make more sugar on sunny days than on cloudy days.

The correct answer is:

d. Plants make more sugar on sunny days than on cloudy days.

Explanation:

When a hypothesis is testable, it suggests that it's reasonable to see whether the forecast is true or not. Testing a hypothesis can include conducting experiments, gathering data and making observations. A useful hypothesis is a testable statement, which may comprise a prediction. A hypothesis should not be associated with a theory.

Explanation :

Hypothesis is a explanation of any process. It can be true or false depending on the outcome of testing.

Out of given hypothesis, correct one is "Plants make more sugar on sunny days than on cloudy days". They prepare their food by the process of photosynthesis.

On a sunny day, plants can easily prepare their food than a cloudy day due to the presence of sun.

So, the correct option is (d).