Which of the following conversions involves change in both shape and hybridisation ?

A group 13 element 'X' reacts with chlorine gas to produce a compound XCl₃. XCl₃ is electron deficient and easily reacts with NH₃ to form Cl₃X $$ \leftarrow $$ NH₃ adduct ; however, XCl₃ does not dimerize X is :

When XO₂ is fused with an alkali metal hydroxide in presence of an oxidizing agent such as KNO₃ ; a dark green product is formed which disproportioates in acidic solution to afforda dark purple solution. X is :

For standardizing NaOH solution, which of the following is used as a primary standard ?