The correct answer is: C. 5th
The Regional Formula was implemented by the Punjab Government in the year 1956, after the 5th Amendment to the Indian Constitution. The 5th Amendment was passed in 1955, and it made a number of changes to the Constitution, including the introduction of the Regional Formula.
The Regional Formula was a system of allocating seats in the Punjab Legislative Assembly to the different regions of the state. It was based on the principle of population, and it aimed to ensure that all regions of the state were represented in the Assembly in proportion to their population.
The Regional Formula was controversial at the time, and it was opposed by some groups in the state. However, it was eventually implemented, and it has remained in place ever since.
The other options are incorrect because they are not the amendments to the Indian Constitution that were passed in the year 1956. The 2nd Amendment was passed in 1952, the 4th Amendment was passed in 1955, and the 7th Amendment was passed in 1959.