In a guitar, two strings A and B made of same material are slightly...

Guitar string i.e. string is fixed from both ends

Frequency $∝ \sqrt{Tension}$
If tension in $B$ slightly decrease then frequency of $B$ decrease.

If $B$ is $536Hz,$ the beats produced will be $536 - 530$ as the frequency decreases, beats with $A$ also decreases.

If $B$ is $524Hz,$ the beats produced will be $530 - 524$ as the frequency decreases, beats with $A$ increases.
$∴$ Original frequency of $B$ will be $524Hz$.