Answer:
Option 3: 

Step-by-step explanation:
Alkynes are of the homologous series of acyclic (alphatic) triple bonded carbon-carbon atoms. The could be branched or unbranched chain, but the triple bonded carbons are ether attached to hydrogen atoms or a single-bond with carbon.
Thus they carry the general formula 
 across the series; where n is any integer greater than 1. The simplest forms of alkynes are acetylene molecules which have the formula
 across the series; where n is any integer greater than 1. The simplest forms of alkynes are acetylene molecules which have the formula 
 .
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I hope this explanation was resourceful and easy to understand.