Assertion:. Human Development Index is not the most reliable measure of human development.
Reasoning: The HDI does not say anything about the distribution of human development.

Both Assertion (A) and reasoning (R) are correct and R is the correct explanation of A.

The correct answer is option 1: Both Assertion (A) and reasoning (R) are correct and R is the correct explanation of A.

Assertion:. Human Development Index is not the most reliable measure of human development. This  is true. While the Human Development Index (HDI) is a revolutionary tool that moved the focus from purely economic growth (GDP) to human well-being, it is widely considered "incomplete" or not the most reliable measure. This is because it is a composite average of three dimensions: life expectancy, education, and per capita income.  
Reasoning: The HDI does not say anything about the distribution of human development. This is also true because HDI does not reflect inequalities or distribution of development among people. The primary technical criticism of the standard HDI is that it is an aggregate measure. It provides a single score for a country but fails to account for internal inequalities. It does not show how these achievements are distributed across different regions, genders, or socio-economic classes within that country. 

Does R explains A? - Yes. This limitation (lack of distribution aspect) is exactly why HDI is not fully reliable. Because the HDI ignores these disparities (the distribution), it can mask deep-seated inequalities. A country could have a high average score while a large portion of its population remains underserved.

NCERT: "The human development index measures attainments in human development. It reflects what has been achieved in the key areas of human development. Yet it is not the most reliable measure. This is because it does not say anything about the distribution."