Alkali metals are sodium, potassium, lithium etc.
On reacting with water , they give metal hydroxide.. M_k (OH)_n
Hydroxide molecular mass = 17..
As molecular mass of the metal hydroxide is 40, n = 1 or 2, and correspondingly the molecular mass of metal is 23 or 6.
That is metal A is either sodium or lithium.
Compound B is : sodium hydroxide or lithium hydroxide.. Lithium hydroxide - molecular mass is not 40..
So metal A is Sodium : Na... compound B is sodium hydroxide.
aluminum oxide: Al₂ O₃ + 2 Na OH +3 H₂O ==> 2 Na Al (OH)₄
Compound C : colourless solution in water ... of compound : sodium tetra aluminate..