The answer is A. Quantitative because Quantitative is observing amount, or quantity. Qualitative would be observing the quality, like color or shape. Hypothetical is making a hypothesis on the size, and finally inferential is would be saying you measured the leaf, I believe. A quick way to solve this would be to see if it involves numbers, and if so it’s a quantitative observation. If it involves color or shape or anything you observe with sight alone, it’s a qualitative observation. I hope this helps and sorry for the long answer, just trying to explain why A is correct.