Solution:
Concept- The molecular formula of the
and 
the number of the homologous series whose first member is ethene is propene and butene. When used for arc welding, ethyne or acetylene is converted into an oxyacetylene flame by mixing it with oxygen gas. In addition, it is frequently used as a fuel and a chemical component. Since it is typically unstable in its pure form, solutions are typically used. Pure ethyne is never smelly. It is a reactive substance that creates a risk of fire and explosion. 2.5% and 93% are its lower and upper explosive limits in air, respectively.The molecular formula of the
and the number of the homologous series whose first member is ethene is Propene and Butene. Homologous differ by a 
unit .The homologues members differ by a
unit so the second and third member are 
i.e propene and butene.Hence, the correct answer is option 1.
