Step 1: Split indicators into two families.
Public health measures are usually grouped as either socio-economic (education, income, living conditions) or health status (how healthy people actually are).
Step 2: Place each option in a family.
Literacy rate, family size, and housing all describe how people live day to day: how educated they are, how big their households are, and what kind of homes they have. These sit firmly in the socio-economic group.
Step 3: Look closely at life expectancy at birth.
This number tells us how long a newborn is expected to live on average. It is shaped by socio-economic conditions, but what it actually measures is a health outcome, so it belongs with the health status indicators, not the socio-economic ones.
Step 4: Conclude.
Since three options measure living and social conditions while life expectancy measures a health outcome, life expectancy at birth is the one that is NOT a socio-economic indicator.