Final answer:
Calcium can interfere with the effectiveness of levothyroxine by binding to the medication, reducing its absorption. For the water-soluble vitamin question, vitamin C is the correct answer as it is the only water-soluble vitamin among the options provided.
Step-by-step explanation:
A nurse educating a client with hypothyroidism on taking levothyroxine should be aware of the interactions that supplements can have with this medication. Among the options given, calcium can interfere with the effectiveness of levothyroxine. It is recommended that calcium supplements be taken several hours apart from levothyroxine because calcium can bind to the medication and reduce its absorption, leading to decreased effectiveness. Answering the water-soluble vitamin question is straightforward since vitamin C is the only water-soluble vitamin among the options listed, which include vitamins A, E, and K. These are all fat-soluble vitamins.