Option 1, Somnath Lahiri, is the correct answer. Somnath Lahiri was the one who warned his colleagues that the Constituent Assembly was British made and was "working the British plan as the British should like it to be worked out".

Option 2, B. R. Ambedkar, is incorrect. While Ambedkar played a significant role in drafting the Constitution of India, he did not provide the warning mentioned in the question.

Option 3, Shyama Prasad Mukherjee, is incorrect. Although Mukherjee was a member of the Constituent Assembly, there is no record of him giving this warning.

Option 4, B. N. Rau, is incorrect. B. N. Rau was a constitutional expert who was appointed as the Constitutional Advisor to the Constituent Assembly. He provided important guidance during the drafting process, but he did not give this warning.

Therefore, the correct answer is option 1, Somnath Lahiri.