Why is income poverty only one measure of counting the 'poor'?

It talks of only one kind of deprivation ignoring all others.
Other deprivations in a human life have nothing to do with lack of purchasing power.
Income poverty is not a permanent condition, it changes from time to time.
Income poverty restricts human choices only at a point of time.

Answer

It talks of only one kind of deprivation ignoring all others.

(a) Correct. Income poverty measures only lack of income/consumption, while poverty is multidimensional and also includes deprivation in health, education, nutrition, housing, sanitation, etc.
(b) Incorrect. Many non-income deprivations are closely linked to lack of purchasing power, even if not fully explained by it.
(c) Incorrect. Poverty may change over time, but that is not the reason income poverty is considered only one measure of poverty.
(d) Incorrect. Income poverty does affect choices, but the core limitation is that it captures only one dimension, not the full range of human deprivation.

Therefore, the correct answer is (a).