Which of the following would be a good question that could be scientifically investigated? what is the best advice a parent can give a child? can playing the piano at a slower tempo improve the sound quality? does lack of sunlight affect plant growth? what is the correct way to pursue happiness in life?

"Does lack of sunlight affect plant growth?"

This question can be investigated with procedures according to
the Scientific Method, because "lack of sunlight" can be arranged
unambiguously, and "affected plant growth" can be identified
unambiguously.

The other questions involve words or phrases with ambiguous definitions,
that can mean different things to different people, and would be hard to
agree on, like  "best advice",  "sound quality",  and  "happiness in life".

Amplify and switch electrical power and electronic signals