Explanation

(c) Suppose $n_{A} =$ known frequency $= 100\ Hz,\ n_{B} = ?$
$x = 5\ bps,$ which remains unchanged after loading
Unknown tuning fork is loaded so $n_{B} \downarrow$
Hence $n_{A} - n_{B} \downarrow = x$ …(i)
$n_{B} \downarrow - n_{A} = x$ …(ii)
From equation (i), it is clear that as $n_{B}$ decreases, beat frequency. ($i.e.\ n_{A} - \left( n_{B} \right)_{new}$) can never be $x$ again.
From equation (ii), as $n_{B} \downarrow$, beat frequency $\lbrack i.e.\ \left( n_{B} \right)_{new} - n_{A}\rbrack$ decreases as long as $\left( n_{B} \right)_{new}$ remains greater than $n_{A}$. If $\left( n_{B} \right)_{new}$ becomes lesser than $n_{A}$ the beat frequency will increase again and will be $x$. Hence this is correct.
So, $n_{B} = n_{A} + x = 100 + 5 = 105Hz$