c
Minimum force required for slipping to occur between $3 \mathrm{kg}$ and
$2 \mathrm{kg}=\mu N_{1}=0.5 \times$ $30=15 N$
Minimum force required for slipping to occur between $2 \mathrm{kg}$ and
$1 \mathrm{kg}=\mu N_{1}=0.3 \times$ $(30+20)=15 N$
Minimum force required for slipping to occur between $1 \mathrm{kg}$ and ground
$=\mu N_{1}=0.1 \times$ $(30+20+10)=6 N$
Thus slipping takes place first at the surface for which minimum force required is least. Thus slipping starts between $1 \mathrm{kg}$ block and ground.