The correct answer is $\boxed{{33\frac{1}{3}\%}}$.
The percentage of work certified is the ratio of the amount of work that has been completed to the total amount of work that is required to be completed. If the amount of work certified is less than 33 1/3% of the contract price, then no profit should be taken to Profit & Loss Account. This is because the contractor has not yet completed enough work to be entitled to a profit.
Option A, 20%, is incorrect because it is less than the required percentage of 33 1/3%.
Option B, 25%, is incorrect because it is more than the required percentage of 33 1/3%.
Option C, $$33\frac{1}{3}\% $$, is the correct answer because it is the required percentage of work that must be completed before a profit can be taken to Profit & Loss Account.
Option D, 40%, is incorrect because it is more than the required percentage of 33 1/3%.