(a)cloudy days:- Due to clouds sunlight will not be able to reach the stomata for photosynthesis and sunlight has an essential role in photosynthesis. So the rate of photosynthesis will decrease.
(b)no rainfall in the area means that there is no water and according to the definition of photosynthesis it is the process by which green plants make their own food using water, sunlight and nutrients. So the rate of photosynthesis will decrease.
(c)Good manuring in the area will increase the rate of photosynthesis as there will be nutrients as well as water in the manure.
(d)Dust on stomata will decrease the rate of photosynthesis as the role of stomata is to trap the sunlight with the help of chlorophyll and if sunlight is not there , photosynthesis can't take place.