A researcher decides to test the scientific validity of the common phrase "an apple a day keeps the doctor away". what would be a valid hypothesis to investigate this specific question?

The correct answer is would be - Eating a single apple every day changes the frequency or number of visiting doctors.

Explanation:

A hypothesis is an explanation or an idea that is based on the observation with prior scientific knowledge which meant to be tested by the experiments and scientific methods.

In this phrase, one can get an idea about the effect of intake of an apple every day affects the numbers of the visits to the doctor it can be hypothesized by two hypothesis-

1) Eating an apple a day does not change the frequency of visiting the doctor. or.

2) Eating an apple on a daily basis decreases the number of visiting doctors.

Thus, the correct answer would be - Eating a single apple every day changes the frequency or number of visiting doctors.