Hello!

Let's break down what is wrong with each of the incorrect answers:

In 1776 Adam Smith wrote that it is young people who have "the contempt of risk and the presumptuous hope of success" needed to found new businesses.

A. who have
This one is correct because it's telling us young people have this particular quality without being overly wordy.

B. with
By changing it to "with," we now have a sentence fragment. If you read it aloud, you'll find yourself wanting to know more information to complete the thought. Young people with these qualities do....what??

C. having
This is also a sentence fragment. What happens as a result of young people "having" these qualities? The sentence doesn't just state that young people have these things, but that there is some effect we need to talk about.

D. who are those with
This is redundant because it uses both "who" and "those," where only one of them is necessary.

E. who are the ones to have
Again, this is redundant. You don't need to say "who" and "the ones" to make your point. They both essentially mean the same thing.

I hope this helps! I'm available if you'd like any follow up.