Member of which groups of algae may be more than 75 meters in length

Explanation

Brown algae, specifically those in the order Laminariales, commonly known as kelp, are known to form large and complex multicellular structures. These algae can grow to impressive lengths, with some species reaching over 75 meters. This makes them the correct answer. Euglenoids are not typically large and do not form multicellular structures. Diatoms, while abundant and ecologically significant, are single-celled and microscopic. Red algae, although diverse, generally do not grow as large as brown algae and are often found in more compact forms.