Fish hatchery workers know that if they want artificially spawned salmon to return to a specific stream to spawn when they reach adulthood, the workers have to raise the fry (baby salmon) in water from that stream during a certain critical time in their early development. this is a clear demonstration of
a. trial-and-error learning
b. operant conditioning
c. imprinting
d. habituation

c. imprinting  

Explanation:

Imprinting is the object of study of ethology, which is related to animal behavior. In the early stage of an animal's life, imprinting is an essential and most conducive phase for learning and subsequent development. The two main types of imprinting are sexual and filial.  In sexual imprinting, a young animal learns the traits it seeks in a mate. Affiliate imprinting is the learning of a offspring from his / her parent.

In the case of the salmon fish situation, the need for workers to raise fingerlings (salmon) in stream water during a certain critical period in their early development is an example of imprinting. In this case, imprinting is necessary for the development of this animal in a different environment from the one it was raised.

it has to be understood that the sea water contains a huge amount of sodium and chlorine ions and if they enter the cell of the atlantic cod fish, the cell will shrivel and the fish will die. to avoid such a condition, the fish uses active transport for the purpose of removing the ions against the gradient.

explanation:

the atlantic cod is not a maritime osmoconformer. its native application of salts is less than that of the seawater it lives in. it is continually falling water from the gills to the ocean via osmosis.

countering that, the fish is going to be swallowing seawater. but since its internal collection of salts is lower than seawater, the fish will want to go free of excess salt, and since that's going upon a gradient, then active transport, requiring energy, must be used to pump out the salt. much marine fish use the organs to filter out the salt and these fish discharge very straight urine.