Final answer:
The answer D, which includes both using as many quadrats as possible and placing quadrats at nonrandom locations throughout the study area, often improves the accuracy of population size estimates based on area-based counts like quadrat counts.
Step-by-step explanation:
The strategy that often improves the accuracy of estimates of population size based on area-based counts such as quadrat counts includes both using as many quadrats as possible (a) and placing quadrats at nonrandom locations throughout the entire study area (c). Using many quadrats reduces the potential for sampling error and gives a better representation of the whole area. Placing quadrats nonrandomly but systematically throughout different parts of the habitat ensures that the sample covers the area efficiently and leads to a better estimate of the population. The answer to the question is D. Both a and c. Additionally, using quadrats for counting individuals in a population can increase the accuracy of the mark and recapture method, although this is more relevant for mobile organisms.