Must most mosquito larvae return to the water surface to obtain air?

Most mosquito larvae must return to the water surface to obtain air because they are not true aquatic organisms and do not possess gills for underwater respiration. Instead, they have a siphon or a respiratory tube that allows them to breathe atmospheric oxygen while floating just below the water's surface. This behavior is critical for their survival since they need to surface regularly to take in air.

The other choices suggest particular circumstances or exceptions that do not apply universally to all mosquito larvae. For instance, while the pupal stage is a transformation phase where mosquitoes do not feed and may rely on the surface for air, it is not a characteristic of larval behavior as a whole. Additionally, the notion that larvae only need to surface in polluted water is misleading, as larvae from healthy water bodies are also obliged to return to the surface for air. This fundamental requirement for respiration accounts for the necessity of accessing the water surface consistently during their larval stage.