b
For first resonant length \(v=\frac{v}{4 \ell_{1}}=\frac{v}{4 \times 18}\) (in winter)

For second resonant length

\(v^{\prime}=\frac{3 v^{\prime}}{4 \ell_{2}}=\frac{3 v^{\prime}}{4 x} \text { (insummer) } \quad \therefore \frac{v}{4 \times 18}=\frac{3 v^{\prime}}{4 \times x}\)

\(\therefore \quad x=3 \times 18 \times \frac{v^{\prime}}{v} \quad \therefore x=54 \times \frac{v^{\prime}}{v} \mathrm{cm}\)

\(\mathrm{v}^{\prime}>\mathrm{v}\) because velocity of light is greater in summer as compared to winter \((\mathrm{v} \propto \sqrt{\mathrm{T}})\)

\(\therefore x>54 \mathrm{cm}\)