Answer=1
C2H2 is correct when writing the molecular formula, but when we write the empirical formula, we use the smallest whole number ratios...
So C2H2 (2 Carbons:2Hydrogens) becomes CH (1Carbon:1Hydrogen)
The other ones have an incorrect empirical formula...
2.C3H4 is already in empirical form, so it should stay C3H4
3. C4H6 in empirical form should be C2H3
4.C5H8 is also already in empirical form, so it should stay C5H8