The correct answer is: A. ${\text{Earned profit}} \times \frac{{50}}{{100}}$
The amount of profit that is normally credited when 50% of the contract is completed is the earned profit multiplied by 50%. This is because the earned profit is the profit that has been earned on the work that has been completed, and 50% of the contract has been completed.
Option B is incorrect because it multiplies the earned profit by 3/2, which is not the correct percentage of the contract that has been completed.
Option C is incorrect because it multiplies the earned profit by 2/3, which is not the correct percentage of the contract that has been completed.
Option D is incorrect because it multiplies the earned profit by 2/3, which is not the correct percentage of the contract that has been completed, and it also divides by the certified value, which is not relevant to the question.