Four point charges are placed at the corners of a square $ABCD$ of side $10 cm,$ as shown in figure. The force on a charge of $1 \mu C$ placed at the centre of square is

Forces of repulsion on $1 \mu C$ charge at $O$ due to $3 \mu C$ charges at A and C are equal and opposite so they cancel out each other. 

Similarly, forces of attraction on $1 \mu C$ charge at $O$ due to $-4 \mu C$ charges at $B$ and $D$ are also equal and opposite. So they also cancel out each other. 

Hence the net force on the charge of $1 \mu C$ at $O$ is zero.