Answer: b. The put price decreases to $3.50
Step-by-step explanation:
Put - Call Parity refers to the relationship that a certain European Put has with a European Call of the same underlying asset, strike price, and expiration date.
If Put - Call Clarity holds then the options and the calls should move together when Volatility changes all else being equal.
In the above scenario, the price of the call DROPPED by $0.5 to $2.50.
This means that the Put Price must DROP AS WELL by $0.5 to $3.50 to maintain the Parity.