What is the mole ratio of d to a in the generic chemical reaction 2a+b-- c+3d?

 D : A is 3 : 2.

Explanation:

The law of definite proportion is being used here. The law states that a given compound always has the same proportion of its constituent elements by mass.  

2a + b = c + 3d

This actually means that, two moles of “a” react with a mole of “b” to produce a mole of “c” and 3 moles of “d”

Using the concept of stoichiometry coefficients of d and a  

d:a = 3:2

c is having 4 valence electrons and is bonded to three hydrogens and one sulphur atom, therefore, the number of bond pair is equal to four. as all the valence electrons are used in forming the bonds so, there is no lone pair left on carbon.

s is having 6 valence electrons and is bonded to one other sulfur atom and one carbon atom, therefore, the number of bond pair is equal to 2. as only two valence electrons are used in forming the bonds so, four electrons are left. therefore, two lone pairs are present on s.