The centre of the square is equidistant from all four corners. The charges at opposite corners A and C are equal ($q_A = q_C = 2 \mu\text{C}$) and positive; therefore, the repulsive forces they exert on the central $1 \mu\text{C}$ charge are equal in magnitude and opposite in direction, cancelling each other out. Similarly, the charges at opposite corners B and D are equal ($q_B = q_D = -5 \mu\text{C}$) and negative; the attractive forces they exert on the central charge are also equal and opposite, cancelling each other out. Consequently, the net electrostatic force on the charge at the centre is zero. (See NCERT Physics Class 12, Exercise 1.6).