Francisco finds a Web site that defines a regular hexagon as a "six-sided polygon with six congruent angles and six congruent sides." Is this a valid definition of a regular hexagon?

a. It is valid because regular hexagons have six sides and six angles.
b. It is not valid because some hexagons that are not regular hexagons fit the definition.
c. It is not valid because it does not say that the figure can only have six angles.
d, It is valid because all regular hexagons fit the definition, and any figure that is not a regular hexagon does not fit the defintion.