What is the transition temperature of alpha- and beta-sulfur?

The correct answer is option 3. 95.9 °C.

The transition temperature between \(\alpha \)-sulfur and \(\beta \)-sulfur is 95.9 °C. \(\alpha \)-sulfur and \(\beta \)-sulfur are two allotropes of sulfur that interconvert at a specific temperature. \(\alpha \)-sulfur (also known as rhombic sulfur) is the stable form at temperatures below 95.9 °C. When heated above this temperature, \(\alpha \)-sulfur transforms into \(\beta \)-sulfur (also known as monoclinic sulfur), which remains stable until the melting point of sulfur is reached. This transition temperature of 95.9 °C is crucial in understanding the phase behavior of sulfur.

Therefore, the correct answer is 95.9 °C.