The correct answer is “if”.
The word “if” is used to introduce a condition or supposition. In this case, the condition is that we do exercise. The supposition is that we will remain healthy.
The word “unless” is used to introduce a condition that must be met in order for something else to happen. In this case, the condition is that we do not do exercise. The supposition is that we will not remain healthy.
The word “when” is used to introduce a time when something will happen. In this case, the time is when we do exercise. The supposition is that we will remain healthy.
The word “more” is used to introduce a greater amount or degree of something. In this case, the greater amount or degree is exercise. The supposition is that we will remain healthy if we do more exercise.
Therefore, the correct answer is “if”.