He wanted three things out of the class: to learn a new skill, to make new friends, and to develop a different mindset.
Explanation:
<u>Three infinitives</u> (to learn, to make, to develop) explain the object <em>three things</em>; that's parallelism, i.e. use of identical grammatical construction.
The other options use both gerund(s) and infinitive(s). Using different constructions cannot create parallelism; that's why those sentences do not contain correct parallel structure.
