we have
C -----> total charge
b -----> number of buckets of balls
Remmeber that
the equation of the line in slope intercept form is equal to
y=mx+b
where
m is the slope and b is the initial value or y-intercept
In this problem
m=$1.50 per buckey
b=$5
therefore
y=1.50x+5
or
C=1.50b+5
answer is first option