A rectangle has side lengths of 3 inches and 5 inches. Which statement must be true about the area of the rectangle?

A. It is rational number because the product of the side lengths is the rational number 15.
B. It is an irrational number because the product of the side lengths is 3/5, an irrational number.
C. It is a rational number because the product of 5 and 5 results in 5, a rational number.
D. It is an irrational number because the product of the side lengths is √15, an irrational number.