Ethene and ethyne are both hydrocarbons containing two carbon atoms.
Ethene ($C_2H_4$) has a double bond between the two carbon atoms and four hydrogen atoms. Its structure is $CH_2=CH_2$.
Ethyne ($C_2H_2$) has a triple bond between the two carbon atoms and two hydrogen atoms. Its structure is $CH\equiv CH$.
Comparing ethene and ethyne:
Number of carbons: Both have 2 carbon atoms, so they don't differ in the number of carbons. *
Number of bonds: Ethene has a double bond, while ethyne has a triple bond. Therefore, they differ in the number of bonds. *
Number of hydrogens: Ethene has 4 hydrogen atoms, while ethyne has 2 hydrogen atoms.
Therefore, they differ in the number of hydrogens. Since ethene and ethyne differ in the number of bonds and the number of hydrogens, the correct answer is (4).
Complete the following reactions by writing the structure of the main products: