Questions

Except 142 all are divisible by number '7'.

The logic in the given puzzle is 

Addition of numeric values of corresponding letters in the first three figures gives respective corresponding letter in fourth figure. 

i.e, A = 1 , W = 23 , B = 2 => 2 + 1 + 23 =26 = 'Z'

Similarly, L = 12, K = 11 , A = 1 => 1 + 11 + 12 = 24 = 'X'

Here the missing letter is 'X'.

Gammaxene, D.D.T. & Bleaching powder are important compounds of Chlorine.

Polo is the sport which is played in the largest pitch.

A piano is a musical instrument which has keys, pedals and strings in which the strings are struck by hammers.