Step 1: Recall HDI components
HDI combines three dimensions: Health (Longevity), Education, and Standard of Living.
Step 2: Indicator for longevity
UNDP measures the health/longevity dimension using life expectancy at birth.
Step 3: Eliminate distractors
Child survival rate, healthy life expectancy, and DALYs are used in other indices/health studies, not in HDI.
Final Answer: (D) life expectancy at birth.