Option B is the right answer.
A parabola is part of a conic section which is got by cutting a right circular cone by a plane, when the intersection gives rise to an open figure.
A parabola is defined as one conic section with eccentricity 1.
Eccentricity is defined as the ratio of distance of the curve from a line to the distance from a point.
In parabola, the line is the directrix, and the point is the vertex.
And always in a parabola, the distance from the directrix will equal the distance from vertex
Hence option b is right.