“An option to buy goods granted to a possible purchaser is, in the strict legal sense, an offer to sell which the offeror promises for valuable consideration…..not to revoke. The owner of the goods, in popular language, is said to have “agreed to sell” them, but in law there is no such agreement unless the parties are mutually bound…… A holder of an option is free to exercise it or not as he may choose, and unless and until he does so, he has neither bought nor agreed to buy. Once an option to buy has been exercised, there is of course a contract of sale of goods within the meaning of the Act.”