A testable question has four properties. Check yours against all four.
Specific. Every noun is defined. "Does music affect plants?" fails; "Does 8 hours of daily 60 dB classical music change the mean height of Vigna radiata seedlings over 21 days?" passes.
Measurable. The outcome is a number or a category you can record with equipment you have. "Healthier" is not measurable; "chlorophyll concentration by SPAD meter" is.
Bounded. The scope fits your calendar. If answering it properly needs a year, you will answer it badly in ten weeks.
Falsifiable. There is a result that would show you were wrong. If no possible outcome would change your mind, it is not a scientific question.
Before: "How do fertilisers help plants grow?" After: "How does nitrogen concentration (0, 50, 100, 200 ppm) affect the 28-day dry biomass of Raphanus sativus grown under controlled light?"
The second is narrower, and it is the narrowness that makes it answerable.