The vertex of a parabola can be:

a. Always a minimum

c. Either a minimum or a maximum

b. Always a maximum

d. Both a minimum and a maximus

c. Either a minimum or a maximum

Step-by-step explanation:

considering that a parabola can grow or decrease from its vertex we can note that in the case of a parabola with the branches down the vertex will always be a maximum, and in the case of a parabola with the branches up it will always be a minimum.