Answer:
C. 
Step-by-step explanation:
Let us assume that,
n is the number of 9-pound cannons and,
t is the number of 12-pound canons.
As captain Two-Pegs has room for 10 cannons, so
---------------------------1
He has allotted for 111 pounds of cannon on his ship, so
-------------------2
i.e 
Therefore, option 3 is the equations that would be a constraint to Captain Two-Pegs maximizing his cannon load.