Final answer:
Excavata is the protist group with mitochondrial remnants called mitosomes, which have reduced functionality and are found in species like Giardia lamblia.
Step-by-step explanation:
The protist group that exhibits mitochondrial remnants with reduced functionality is the Excavata. Specifically, among the Excavata, the diplomonads, such as Giardia lamblia, possess mitochondrial remnant organelles known as mitosomes. While these mitosomes are not functional as respiratory organelles, they do play a role in iron and sulfur metabolism. Diplomonads have adapted to anaerobic environments and rely on alternative pathways, like glycolysis, for energy production.