The bacteria on the exoskeleton provide nutrition to the ants is the research hypotheses most likely to be correct. The correct option is 'C'.
Step-by-step explanation:
Information given shows:
Leaf cutter ant harvests leaf.
When brought to nest fungi (Leucoagaricus) is grown and eaten by ants.
Another fungal (Escovopsis) kills Leucoagaricus when ant is not around. This means that bacteria in the exoskeleton of ant prevents the attack or harm caused by Escovopsis. The secondary metabolite produced by bacteria prevents the fungal attack.
This shows that bacteria is in mutualistic relation with the ant. The presence of ant is the presence of bacteria in exoskeleton which saves the food source of ant which was fungal Leucoagaricus.