we know that
A relationship between two variables, x, and y, represent a proportional variation if it can be expressed in the form y=Kx
where
K is the constant of proportionality or slope of the linear equation
In this problem
x ----> is the number of months
y -----> is the total amount savings
we have the ordered pairs
(4,80) and (7,140)
so
Find the value of K
K=y/x
For (4,80)
K=80/4=20
For (7,140)
K=140/7=20
The value of K is the same for both ordered paits
that means------> is a proportional relationship
therefore
the linear equation is
y=20x
the answer is the option A, C and D