what is a common feature of all protists?

they are prokaryotic.
they absorb nutrients.
they require moisture.
they can photosynthesize

They require moisture  is a common feature of all protists

Explanation:

Protists are categorized based on how they eat, how they move and how similar they are to the other eukaryotic kingdoms. Protists are single-celled almost. One can speculate regarding protists as all eukaryotic organisms that are not animals, neither plants not a fungus. They also may seek for food like an animal or photosynthesize like a plant.

Protists are organisms that need moisture to live. They mostly live in aquatic habitats, though they may also grow in moist tree trunks and soil, among other environments. Protists also go through periods of sexual or asexual reproduction, based on their species or seldom their environmental situations.