Which is the correct use of the word “emerged”?
west african kingdoms emerged to form one union.
trade emerged as a main source of income.
gold emerged from one merchant to another.
which merchant will best emerge the wealth?

The correct answer is: Trade emerged as a main source of income.

The verb "to emerge" means, in this scenario, becoming apparent, important. So it would mean that now, trade has become the main source of income.

Also, all the other sentences are not making the most appropriate use of the verb, being best substituted by other verbs.

utilized the penal colony