Greetings Aspirant,

If the magnetic field along AB is  B , the force acting on BC=F=BiL

Now the length of the side AC is  Root"2L"

Also the current reverses the direction in this section as compared to that in BC.

Hence force on AC= Root"2"L( B× I )

= − Root"2"BILsin(45)

=−BIl=− F

If you are unable to understand the Answer I have uploaded an Image for Better Understanding.

https://drive.google.com/file/d/1e6gsucWKEXxS1si-XlTXIEmc11SRBq8K/view?usp=sharing

Hope My Answer Helped You.

Best Of Luck For Future.