Algae that form motile colonies are:
The correct option is B.
Explanation of correct option:
Explanation of incorrect options:
Nostoc is a cyanobacteria, not an alga.
Spirogyra forms long, filamentous colonies that are not motile.
Chlamydomonas form microscopic unicellular colonies that are not motile.
Final answer: Algae that form motile colonies are Volvox.