d
According to question, tuning fork gives \(1\, beat/second\) with \((N)\, 3^{rd}\) normal mode. Therefore, organ pipe will have frequency \(\left( {256 \pm 1} \right)\,Hz\). In open organ pipe, frequency
\(\mathrm{n}=\frac{\mathrm{NV}}{2 \ell}\)
or, \(255=\frac{3 \times 340}{2 \times \ell} \Rightarrow \ell=2 \mathrm{m}=200 \mathrm{cm}\)