What is the nature of $Al_2O_3$?

Given below are two statements :
Statement $I$ : Gallium is used in the manufacturing of thermometers.
Statement $II$ : $A$ thermometer containing gallium is useful for measuring the freezing point $(256 \ K)$ of brine solution.
In the light of the above statement,choose the correct answer from the options given below :

Which of the following statements about $H_3BO_3$ is not correct?

The weakest Lewis acid is

Which of the following substances imparts a green color to the burner flame?

The incorrect stability order of $+3$ and $+1$ states of $13^{th}$ group elements (boron family) is