Correct Answer - C
`4xx` side `=40cm` brgt (given)
`implies` side `=40/4=10cm`
In `DeltaAOB`
`OB=sqrt(10^(2)-6^(2))=sqrt(100-36)`
`=sqrt(64)=8cm`
Diagonal `BD=8xx2`
`=16cm`
Alternative
समचतुर्भुज की भुजा
`=1/2sqrt(d_(1)^(2)+d_(2)^(2))`
`10=1/2sqrt(12^(2)+d^(2))`
`20=sqrt(144+d_(2)^(2))`
`144+d_(2)^(2)=400`
`d_(2)^(2)=400-144=250`
`d_(2)=sqrt(256)=16cm`