The question would have been clearer if it had stated
A(L)=5L
according to standard math notation indicating that L is an independent variable. 5 is a numeric value, and therefore cannot vary, therefore a constant.
A constant of proportionality k when used in a proportional relationship, written
F(x)=kx
means that the function is proportional to the value of the independent variable x.
In the given case,
A(l)=5l, so 5 takes the place of, or simply IS, the constant of proportionality.
Note that the lowercase L in the above equation may have been, or could have been, confused with the numeric digit one (1). I usually write in uppercase when the letter L is used as a variable to avoid this problem.