Which type of quadrilateral has exactly one pair of parallel sides?

Any type of trapezoid does.

The Trapezoid is a parallelogram with 1 pair of parallel sides

a complex number is a number that can be expressed in the form a + bi, where a and b are real numbers, and i is a solution of the equation x2 = −1. because no real number satisfies this equation, i is called an imaginary number. has no real solution, since the square of a real number cannot be negative.