Final answer:
The difference between Cladophora and Chaetomorpha in their holdfasts is that Cladophora has a branched holdfast while Chaetomorpha has a single, unbranched holdfast.
Step-by-step explanation:
The main difference between Cladophora and Chaetomorpha in terms of their holdfast is that Cladophora typically exhibits a branched or rhizoidal holdfast, which helps it attach to substrates in various aquatic environments, whereas Chaetomorpha tends to have a simple or unbranched holdfast. The holdfast of Chaetomorpha is not commonly differentiated into a distinct structure like the branched forms seen in Cladophora, making answer A the correct choice: Cladophora has a branched holdfast, while Chaetomorpha has a single holdfast.