Let's say that the dogs eat 50 lbs of food in 50 days. That would mean they eat 50/50 = 1 pound per day. Notice I divided the weight over the number of days. The dog eats 1 pound per day, for 365 days, so in total it eats 365*1 = 365 pounds. This is just a hypothetical example.
In general, if x is the number of days to eat 50 pounds of food, then x/50 is the expression representing the amount eaten per day (in pounds). Finally 365*(x/50) is the expression representing the total amount eaten over the whole year.
This is why the final answer is choice C