Choice "C" is correct. Specific performance is a court order to perform under the terms of a contract. Generally, it is available only in contracts for unique or rare property. A patent, by definition, is unique. Therefore, specific performance would be available to enforce a contract for the sale of a patent.
Choice "b" is incorrect. Specific performance is not available to enforce a contract of employment, at least not in an action by the employer, because it would be tantamount to an order of involuntary servitude, which is prohibited by the 14th Amendment to the United States Constitution. Choice "a" is incorrect. Specific performance is available only to enforce a contract involving unique or rare property. It is not available to enforce a contract for fungible goods, as such goods are easily replaced.
Choice "d" is incorrect. Specific performance is available only to enforce a contract involving unique or rare property. It is not available to enforce a contract for fungible items. Stock traded on a national securities exchange is fungible.