The correct answer is: conflict.
A conflict is a disagreement or argument, typically a prolonged one. In this case, the conflict is between the Election Commission and the Union Government over the powers of the former in respect of the deployment of central police forces at places where an election is held. The Election Commission believes that it has the sole authority to deploy central police forces, while the Union Government believes that it has the right to do so as well. This conflict has led to a number of disputes between the two bodies, and has also raised concerns about the independence of the Election Commission.
The other options are incorrect because they do not accurately describe the nature of the relationship between the Election Commission and the Union Government. Option B, pain, is too vague. Option C, irritant, suggests that the relationship is simply annoying, when in fact it is much more serious than that. Option D, culprit, suggests that one side is to blame for the conflict, when in fact both sides are responsible.
In conclusion, the correct answer is: conflict.