Why does king leodegrance give king arthur the round table?

a.
it is payment for king arthur's lending him 30 knights.

b.
it is a present to honor arthur's becoming king of the britons.

c.
it is a reward for king arthur's saving cameliard.

d.
it is a dowry for king arthur and guinevere.

The answer is D. 

It is a dowry for the King and Guinevere. 
In the original story Guinevere is king Leodagrance's daughter and so when arthur and her get married the table was given as a gift.

In Merlin, Gwen (Guinevere) is a servant for the Lady Morgana, (King Uthers ward). Even though she is still a servant, she marries Arthur and Becomes the Queen of Camelot. 

Hope this helps.