Answer: a. By painting him on horseback, any imperfections were easily minimized.
Step-by-step explanation:
The painting in question features King Phillip IV on horseback and was most probably done to minimize any deficiencies or imperfections that would appear if he was standing.
Kings in the past (and indeed the present) needed to portray a certain image of strength and grace which was why the way they were to appear to the public needed to be scrutinized. Portraits are an example of this which was why Philip had to be painted in such a way as to minimize imperfections.