Step 1: Concept Overview:
The question requires identifying a parasitic algal genus that grows intercellulary within a host plant.
Step 2: Detailed Analysis:
Analyzing the lifestyles of the provided algal genera:
(A) {Cladophora:} Typically a free-living filamentous green alga found in freshwater or marine habitats. It is non-parasitic.
(B) {Chlamydomonas chrenbergii:} A free-living, unicellular, motile green alga, commonly inhabiting soil and freshwater. It is non-parasitic.
(C) {Cephaleuros:} A known parasitic green algal genus. {Cephaleuros virescens}, for example, parasitizes terrestrial plant leaves (tea, coffee, mango), causing "red rust" disease. Its filaments grow in the subcuticular and intercellular spaces of the host.
(D) {Protoderma:} A green algal genus forming crusts on substrates like rocks or other plants (epilithic or epiphytic). It is non-parasitic.
Step 3: Conclusion:
{Cephaleuros} is the intercellular parasitic alga from the given options.