Which group of protozoans is motile by flagella?

Mastigophora is the correct answer because this group of protozoans is characterized by their use of flagella for movement. Flagella are whip-like structures that help these organisms swim in their aquatic environments. Members of the Mastigophora, such as euglena and trypanosomes, exhibit various forms of flagellation, allowing them to be motile and navigate through different media efficiently.

In contrast, Sarcodina represents amoeboid protozoans that primarily move by extending pseudopodia, which are temporary projections of their cytoplasm. Apicomplexa, on the other hand, are mostly non-motile and include parasitic forms, relying on hosts for transmission rather than locomotion. Ciliophora is characterized by the presence of cilia, which are short hair-like structures that facilitate movement but differ fundamentally from the flagella used by Mastigophora. Therefore, the defining characteristic of Mastigophora being motile by flagella sets it apart as the correct option.