How do we approach this question?

A high conductivity means more dissociation of acid in ions or a strong acids.
Low conductivity means less dissociation of acid meand weak acid.
In the given list H₃PO₄ is a weak acid so sample 2 is H₃PO4.
H₂SO₄ and HCl are strong acids with same moles ( Molarity and volume is same) and H₂SO₄ gives exactly same H + ions as HCl after dissociation of first H.However it also undegoes second H dissociation (not fully) so H+ ions and conductivity for H2SO4 is more than HCl .Sample  1 HCl and sample 3 is H₂SO₄.
b(i) CH₃NH2 is a weak base so it will be reatined in netionic reactions and HCl is strong acid fully dissociated.All salts of ammonium or ammonium type are completely soluble so strong electrolytes.
CH₃NH₂  + H⁺    +   Cl^-
        ----->  CH₃NH₃⁺ + Cl^-
Net ionic reation is
CH₃NH₂ + H⁺--->CH₃NH₃⁺

(ii)  Make RIF table of the reaction and check the what all is left and work out the pH based on it.
CH₃NH₂  + H⁺    +   Cl^-
3.0 mmol              2.0mmol
        ----->  CH₃NH₃⁺ + Cl^-

Clearly CH₃NH₂ is in excess and HCl is Limiting reactant .Make complete RIF table and you will find finally CH₃NH₂ and its conjugate are present in the solution.
Use buffer solution concept and Handeson and Hasselbach equation.