Final answer:
Among the given options, cacti are the biotic factor in a desert ecosystem because they are living organisms.
Step-by-step explanation:
The question is asking to identify which option is a biotic factor in a desert ecosystem. A biotic factor refers to any living component that affects the population of another organism, or the environment.
Given the choices: temperature fluctuations (abiotic), sand dunes (abiotic), cacti (biotic), and sunlight exposure (abiotic), the correct answer is cacti, as they are living organisms and therefore biotic components of the desert ecosystem.