Answer:
The correct answer is: His curiosity and skepticism, two well-known traits of his, lead him to conduct more research.
Explanation:
In the example above, we use commas in order to highlight and separate the appositive which renames the nouns besides it, in this case: his curiosity and skepticism. According to the grammar and orthography rules, we separate this appositive with a comma, as the nouns preceding the appositive provides sufficient identification on their own.
We should also add a dash between the words well and known to join these two adjectives before a noun (in this case: traits). If these adjectives were located after a noun, we wouldn't use a dash.
