Suggestions or answers ?

HYPOTHESIS: IF the salinity of the solution decreases, THEN the vegetable mass increases.

Explanation:

Hypothesis, in an experiment, is a testable explanation or prediction made to solve an observed problem or answer a question. It shows how one variable called independent variable affects another variable called dependent variable. It is usually written in the "IF, THEN" format.

In this case, the question states that there is a relationship between vegetable mass and salinity of the solution. This means that the question has been asked that "Why is the vegetable's mass affected by salinity of a solution". Hence, a hypothesis can read:

IF the salinity of the solution decreases, THEN the vegetable mass increases.