The elements of this, and any other, contract include practicality.
Option d
Explanation:
From the above statement the companies that enters into a contract for completing tasks has consideration upon completion of tasks. While entering a contact two things will happen that is payments and another important thing is the law of contrast.
Actual reason for the contracts to happen is the practicality as the job to be done and the deal between both the companies are practically possible. Moreover, it is a benefit for both parties. Here the task of construction of payment is compensated by the payment on completion of work.