a) The clothing the subject is wearing shows he is most likely the artist showing his skill. The clothes are not high end so he is not trying show off his wealth.
b) The background objects are most likely to show off his skills or to show his religion. The painting is called "Self Portrait as the Apostle of Paul." The Apostle Paul, originally known as Saul of Tarsus, was an Apostle who taught the gospel of Christ to the first-century world. The artist may be reading his teaching or is trying to resemble him in some way.
c) The artist most likely wants to show his faith and skill in the arts. The artist shows chiaroscuro which can be very complicated because it shows the light and darkness around them. The artist expresses how he is very serious about his religion.
d) I have heard of the artist and this is a self portrait. The painting does give you a sense of the artist's personality. The artist likes to walk in other people's shoes. The title says "as the Apostle Paul" which means he is trying to show himself as the Apostle Paul.