1. Concept of Young's modulus

   Young's modulus $Y$ is defined as $Y = \frac{\text{stress}}{\text{strain}}.$ It measures the stiffness of a material.

2. Effect of temperature on solids

   When temperature rises, the atoms/molecules of a solid vibrate with greater amplitude about their mean positions.

3. Reason for change in elasticity

   Due to increased thermal vibrations, the interatomic restoring forces become effectively weaker for small deformations. Hence the material becomes slightly less stiff.

4. Conclusion

   Therefore, with increase in temperature, the Young's modulus generally decreases.

5. Option check

   • A: changes erratically — Incorrect
   • B: increases — Incorrect
   • C: decreases — Correct
   • D: remains unchanged — Incorrect

So the correct answer is C.