1. Use the relation for frequency of a tuning fork

For a tuning fork, the frequency depends on the elastic property and density of the material. Roughly,

$f \propto \sqrt{\frac{E}{\rho}}$

where:

• $E$ = Young’s modulus of the material
• $\rho$ = density of the material

2. Effect of increase in temperature

When temperature increases:

• Young’s modulus $E$ generally decreases for metals.
• Density $\rho$ may also change slightly due to expansion, but the dominant effect is the decrease in $E$.

Since frequency is proportional to $\sqrt{E/\rho}$, a decrease in $E$ causes the frequency to decrease.

3. Conclusion

Therefore, when temperature increases, the frequency of a tuning fork decreases.

4. Check the options

• A: increases $\to$ Incorrect
• B: decreases $\to$ Correct
• C: remains same $\to$ Incorrect
• D: increases or decreases depending on the material $\to$ Incorrect for the standard JEE result

Hence, the correct answer is:

$\boxed{\text{B: decreases}}$