The correct answer is option 'A', which states that the right of joint promises to demand performance is joint.

Explanation:
When two or more parties make a joint promise, it means that they are collectively bound to perform a particular obligation. In other words, all the parties involved are equally responsible for fulfilling the promise.

The term "joint" refers to the fact that the rights and obligations arising from the promise are shared by all the parties involved. This means that any one of the parties can demand performance from the other party/parties. Each party has an equal right to enforce the promise and hold the other party/parties accountable for their share of the performance.

In contrast, the term "several" would mean that each party is individually responsible for their own share of the promise. In such a case, the parties would not have the right to demand performance from each other. Instead, they would only be able to enforce their own individual obligations.

Option 'C', which states that the right of joint promises to demand performance is joint or several, is incorrect. The right is specifically joint, meaning that it cannot be several.

Option 'D', which states that the right of joint promises to demand performance is joint and several, is also incorrect. While joint and several liability can apply to certain legal situations, it does not apply in this context. Joint and several liability would mean that each party is individually and collectively responsible for the fulfillment of the promise. However, in the case of joint promises, the right to demand performance is solely joint, not joint and several.

To summarize, when parties make a joint promise, they have the right to demand performance collectively. This means that any one of the parties can enforce the promise and hold the other party/parties accountable for their share of the performance.