Final answer:
Leaf cells contain more chloroplasts than other plant cells to optimize photosynthesis, which requires sunlight. Sunlight is the critical abiotic factor for this adaptation, as it is directly used in photosynthesis.
Step-by-step explanation:
Leaf cells have more chloroplasts than other cells within a plant primarily because chloroplasts are the site of photosynthesis, the process by which plants convert sunlight into chemical energy. The most important abiotic factor that makes a high number of chloroplasts in leaf cells optimal is sunlight. This is because the upper surfaces of leaves (adaxial surfaces) typically receive more sunlight, which is essential for photosynthesis, while the lower surfaces (abaxial surfaces) tend to be shaded. As a consequence, plants have adapted to maximize light absorption by having more chloroplasts where light is most abundant. Moreover, transpiration rates are higher when more sunlight is available, which can lead to faster water loss but also aids in nutrient uptake and cooling the plant.
Reviewing the provided statements, it's clear that the abiotic factor most pertinent to the question is sunlight (a) since it's directly related to the function of chloroplasts and photosynthesis. Additionally, concerns about transpiration rates help indicate how plants manage water loss while still ensuring that photosynthesis occurs efficiently.