ANSWER: B. (a+b, c)
Since the plane is a parallelogram, if c is the y-value of Point P, then c is also the y-value of Point Q, the corresponding vertex parallel to x-axis.
If the x-value of Point P is a units to the right of y-axis, then add a units to the x-value of its opposite vertex Point R which is b, to find the x-value of Point Q.
Point Q y = c
x = a + b
Coordinates of Point Q = (a + b, c)