Swarmer cells are A. formed by Myxobacteria. B. formed by sheathed bacteria. C. also known as coliforms. D. part of the green nonsulfur bacteria.

Option B, sheathed bacteria

Explanation:

Swarmer cells is basically a stage of lifecycle of bacteria during which the bacteria exists in free swimming form known as swarmer cell stage. In this stage, the cells are not sheathed but they get sheathed in later stage thereby forming sheathed bacteria. Usually these sheathed bacteria grow as long filaments and are covered by sheath from the exterior side. In swarmer cell stage, the cell length increases along with number of flagella per cell.

hence, option B is correct.