A call option gives you a right at a certain point of time to buy existing shares in a company at a predetermined price.
A call option is usually designed as a security and as such the benefit is taxed as ordinary income at the time of acquisition. They are usually transferable and a subsequent sale is taxed as capital gain.
An exercise of the call option to buy a share does not result in any taxation. The sale of the share or the option will be taxed as capital gain.