Answer:
Answer is: A(Linear)
Step-by-step explanation:
A linear function is a function whose graph is a straight line.
It is given that Mary plotted a graph and she found out that the difference in the y values is the same. So such a thing could happen when a function is a linear function.
since, a linear equation is given as:
where a and b are real numbers.
so for distinct values of x say
we get the y-values as 
now,

so for equally spaced
we get the difference in y-value to be equal.
Hence, the function is Linear.