26 C.F.R. § 1.544-4
The shares of stock which may be acquired by reason of an option shall be considered to be constructively owned by the individual having the option to acquire such stock. For example: If C, an individual, on March 1, 1955, purchases an option, or otherwise comes into possession of an option, to acquire 100 shares of the capital stock of M Corporation, such 100 shares of stock shall be considered to be constructively owned by C as if C had actually acquired the stock on that date. If C has an option on an option (or one of a series of options) to acquire such stock, he shall also be considered to have constructive ownership of the stock which may be acquired by reason of the option (or the series of options). Under such circumstances, C shall be considered to have acquired constructive ownership of the stock on the date he acquired his option. For the restriction on the applicability of the rule of this section, see paragraph (b) of § 1.544-1 .
26 C.F.R. §1.544-4