Answer:
B. Decrease to its carrying capacity
Step-by-step explanation:
Goby fish feed over algae for their survival and growth but if 50% of algae in the ocean decreases then the population of goby fish will decrease to its carrying capacity.
This is so because then the goby fish will be dependent on the left number of algae and maximum population size will depend on the rest 50% of food or algae available.
Hence, the correct answer is "B. Decrease to its carrying capacity".