What did king solomon do to make israel a strong and rich nation?

a. he split the twelve tribes of israel into two separate kingdoms.
b. he united the twelve tribes of israel under one leader.
c. he made jerusalem the capital and religious center of israel.
d. he created a large, powerful army and improved transportation.

D. He created a large, powerful army and improved transportation.

Rule out choice A, because at Solomon's time the tribes of Israel were united.  Choices B and C, while things that were true at Solomon's time, were both items accomplished by Solomon's father, King David.

Solomon did increase the power of Israel's military.  For instance we're told that "Solomon had 4,000 stalls for horses and chariots and 12,000 horsemen, and he stationed them in the chariot cities and with the king in Jerusalem. He was the ruler over all the kings from the Euphrates River even to the land of the Philistines, and as far as the border of Egypt" (2 Chronicles 9:25-26).

Solomon also pursued many building projects, including those that aided transportation and commerce.  He is believed to have built up the port of Ezion-Geber in the south, to improve commerce by sea.  As the Bible narrative reports it, "King Solomon also built a fleet of ships in Ezion-Geber, which is near Eloth on the shore of the Red Sea, in the land of Edom.  And Hiram sent his servants with the fleet, sailors who knew the sea, along with the servants of Solomon. They went to Ophir and took four hundred and twenty talents of gold from there, and brought it to King Solomon" (1 Kings 9:26-28).