Based on figure 21-1, which of the major groups of protists is most closely related to plants? a) amoebozoa b) excavates c) rhodophyta d) choanozoa

I believe the answer is Rhodophyta.
Classification of protists in one kingdom is difficult because protists more closely resmble members of other eukaryotic kingdoms than they do other protists. Rhodophyta orred algae are distinct eukaryotic lineage characterized by the accessory photosynthetic pigments phycoerythrin, phycocyanin and allophycocyanins arranged in phycobiliomes and the absene of flagella and centrioles. 

The higher you go up the less oxygen there is