In the context of performance of reciprocal promises, the option that is not applicable is option B - "They cannot be performed simultaneously."

Explanation:

Reciprocal promises refer to a situation where two or more parties promise to do something for each other. In such cases, the performance of these promises can be carried out in various ways. Let's examine the given options to understand which one is not applicable.

a) They can be performed simultaneously:
Reciprocal promises can be performed simultaneously when both parties agree to perform their obligations at the same time. For example, if Party A promises to deliver goods to Party B and Party B promises to make the payment upon delivery, they can agree to perform their promises simultaneously.

c) They can be performed in an order fixed by the parties:
Reciprocal promises can also be performed in an order fixed by the parties. This means that the parties can agree on a specific sequence or order in which they will perform their promises. For example, if Party A promises to provide a service to Party B and Party B promises to pay after receiving the service, they can agree on a specific order of performance.

d) The order of performance may be decided by the nature of the transaction:
The order of performance of reciprocal promises can also be decided by the nature of the transaction. In some cases, the nature of the transaction may require one party to perform their promise before the other party can fulfill their obligation. For example, if Party A promises to provide raw materials to Party B and Party B promises to manufacture goods using those materials, it is logical for Party A to perform their promise first.

b) They cannot be performed simultaneously:
This option is not applicable because reciprocal promises can indeed be performed simultaneously, as explained in option a. Parties can agree to perform their promises at the same time, especially when the promises are interdependent and require simultaneous action.

In conclusion, option B - "They cannot be performed simultaneously" is not applicable in relation to the performance of reciprocal promises. Reciprocal promises can be performed simultaneously, as well as in an order fixed by the parties or based on the nature of the transaction.

Reciprocal promises are those which needed to be performed one by one, without first party's performance other is not able to perform his.