Final answer:
Reduced amounts of xanthophylls in the thylakoid membrane would likely lead to decreased efficiency of photosystem II, affecting the plant's ability to conduct photosynthesis efficiently.
Step-by-step explanation:
If a mutation occurred that reduced the amount of xanthophylls in the thylakoid membrane, the most likely prediction is 1) Decreased efficiency of photosystem II (PSII). Xanthophylls, which are carotenoid pigments, play a significant role in the light-harvesting complex of PSII, which is embedded in the thylakoid membrane. These pigments help in the absorption of light and protect the photosynthetic apparatus from damage caused by excess light. Therefore, a reduction in xanthophylls would likely impair the efficiency of PSII, leading to decreased effectiveness in capturing light energy and transferring excited electrons to the electron transport chain. This may result in a reduced rate of photosynthesis, as PSII is integral in splitting water molecules to release oxygen and provide electrons for subsequent reactions.