In the following question, the sentence given with blank to be filled in with an appropriate word. Select the correct alternative out of the four and indicate it by selecting the appropriate option.
Arun's financial ________ has helped him earn a fortune on the stock market.

Let's consider the words in the context of the financial/stock market domain:
expertise: Expert skill or knowledge in a particular field. "Financial expertise" is a highly precise and standard collocation describing professional knowledge that leads to market success.

dexterity: Skill in performing tasks, especially with the hands (physical agility).

readiness: The state of being fully prepared.

knack: A tendency or an acquired special talent (more casual than professional expertise).

While knack is possible colloquially, expertise is the most professional and fitting word for earning a massive fortune systematically in a stock market.