The form of the linear equation is

y must have power 1 and also x mast have power 1
Let us check each answer

Since x and y have the power of 1, then
The graph of it is a line

Since x and y have the power of 1, then
The graph of it is a line

Since x and y have the power of 1, then
The graph of it is a line

Since x has a power of 2, then
The graph of it is not a line
The answer is D