Final answer:
Freshwater fish have adapted to live in hypotonic conditions by taking in salt and excreting diluted urine.
Step-by-step explanation:
Freshwater fish live in an environment that is hypotonic to their cells. Therefore, they do not prefer hypotonic conditions, but rather, they have evolved osmoregulatory mechanisms to cope with it.
These fish actively take in salt through their gills and excrete a lot of very diluted urine to balance their internal conditions. In contrast, saltwater fish reside in hypertonic conditions and must deal with the opposite situation, secreting salt through their gills and producing highly concentrated urine to maintain osmotic homeostasis. Consequently, the answer to the student's question is (a) Hypotonic.