Cindy Works for Sky Manufacturers,a Public Corporation

Cindy works for Sky Manufacturers,a public corporation.In 20X1 she was offered an option to purchase shares at $15 per share from her employer.The fair market value on that day was $17 per share.The option had a four year exercise time-limit.Cindy exercised her option in 20X3 and purchased 500 shares.The fair market value at that time was $21 per share.What is Cindy's tax treatment of this option in the year 20X3?

A) $1,000 taxable benefit
B) $2,000 taxable benefit
C) $3,000 taxable benefit
D) Not taxable until the shares are sold.