A farmer has 300 m of fencing to construct a rectangular pen with one side bordering a straight river, so no fencing is needed along the river. The two sides perpendicular to the river each have length x, and the side parallel to the river has length y. To maximize the enclosed area, what should be the length of the side parallel to the river?
a) 100 m
b) 75 m
c) 150 m
d) 225 m