Step 1: Draw the figure and mention the direction of forces at B.[Ref. Fig.]
Step 2: Calculate the force experienced by B due to other charges
Applying Coulomb's law,
Force on B due to charge at A,FA=14π∈0Q2a2
Force on B due to charge at C,FC=14π∈0Q2a2
Force on B due to charge at D,FD=14π∈0Q22a2
Step 3: Net resultant force on B
Resultant of FA and FC,F′=√F2A+F2C
=√14π∈0Q2a2+14π∈0Q2a2
=√24π∈0Q2a2
Net force at B=F′+FD {Both are in the same direction}
=√2Q24π∈0a2+14π∈0Q22a2
=(2√2+12)Q24π∈0a2 =(2√2+12)KQ2a2
Hence, Option(C) is correct.