B the purchase cost of the aquarium
Step-by-step explanation
we have the equation

this is a linear equation in the form

hence

so, if we have the sentence
the cost in dollars, y, of purchasing and maintaining a classroom aquarium for x months
let
cost in dollars = y
as x is the variable, let x for the variable in the text, in this case, what changes is the number of months,so
number of months= x
the factor of x ( 48), is the value for the mainting the classroom ( as we can see it depends on the number of months , x)
so
the monthly cost of the aquarium=48
finally , we have 250, this values is a constant, it does not depend on the number of months, so it is
the purchase cost of the aquarium=250
so,the answer is
B the purchase cost of the aquarium
I hope this helps you