Step 1: We need the rarest contributor to infant death from the four choices. Birth injuries (option c) sit at the bottom of this list.
Step 2: Infant deaths in India are driven mainly by infective conditions, prematurity and low birth weight. Sepsis, pneumonia and diarrhoeal disease, together with preterm birth complications, make up the bulk of the toll.
Step 3: Congenital malformations are a recognised but smaller cause, still ahead of mechanical birth trauma in frequency.
Step 4: Better antenatal monitoring, institutional deliveries and skilled birth attendance have cut down injuries occurring during labour, so birth injuries remain the least frequent of the four. The other three options rank higher and are eliminated.
\[\boxed{\text{Birth injuries}}\]