Which group is incorrectly paired with its description?
a. diatoms—important producers in aquatic communities
b. red algae—eukaryotes that acquired plastids by secondary endosymbiosis
c. apicomplexans—unicellular parasites with intricate life cycles
d. diplomonads—unicellular eukaryotes with modified mitochondria

b. red algae—eukaryotes that acquired plastids by secondary endosymbiosis

Explanation:

Red algae did not acquire plastids by secondary endosymbiosis and thus, option b is an incorrect pair. Red algae are eukaryotes that are red in the color not because they acquired plastids y endosymbiosis but because they produce the enzyme phycoerythrin and phycocyanin which masks the other pigments present in it.Secondary endosymbiosis is a process by which the product of primary endosymbiosis is engulfed by another organism. Such type of process never occurred in case of red algae.