Which characteristics do estuaries exhibit? 

A)  constant temperature and variable salinity 
B)  constant temperature and salinity 
C)  variable temperature and salinity 
D)  variable temperature and constant salinity

Estuaries are bodies of water where freshwater from rivers or streams meets saltwater from the ocean, resulting in a mixture of both freshwater and saltwater. This mixture causes the temperature and salinity to vary constantly within the estuary, making option A and B incorrect. Therefore, the correct answer is option C) variable temperature and salinity.