Why is the methodology adopted in India to count the 'poor' debatable?

Answer

It is based on the proposition of poverty as meagre income or buying capacity.

(a) Poverty line in India has indeed been debated, but this is not the core methodological criticism asked here. The main issue is the narrow basis used to define poverty. Verdict: Not the best answer.
(b) Rural-urban diversity exists, and separate poverty lines have been used, but diversity alone does not explain why the methodology itself is debatable. Verdict: Not correct as the main reason.
(c) Absence of a uniform global standard is true in general, but India’s poverty-counting debate is about its own method, not global comparability. Verdict: Not correct.
(d) India’s poverty estimation has been criticized because it largely treats poverty as inadequate income/consumption or purchasing capacity, ignoring multidimensional deprivation. Verdict: Correct.