The element which exists in a liquid state for a wide range of temperatures and can be used for measuring high temperature is-

1. B

2. Al

3. Ga

4. In

Hint: Element has lowest melting point atomic number is 31

In gallium, the crystal structure is different, suggesting that Ga consists of an almost discrete Ga2 molecule, which is why its melting point is the lowest.

Ga exists as a liquid from $30°\mathrm{C}$ up to $2000°\mathrm{C}$ and hence, it is used in high-temperature thermometry.