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We need to give the IUPAC name of below given compound:

Given compound has two carbon atom and a double bond between them. As alkenes with two or three carbon atom need not specify position of double bond so IUPAC name of given compound is ethene.

We need to give the IUPAC name of below given compound:

• Selecting longest carbon chain containing double bond: So in given structure we have longest chain of three carbons atoms named as propene.
• We need to number carbon atoms starting from atom nearer to double bond. Now name is $1$- propene.
• We have substituent at $2$nd carbon atom. Substituent is methyl. So name of compound became$2$- methyl-$1$- propene.

We need to give the IUPAC name of below given compound:

• Selecting longest carbon chain containing triple bond: So in given structure we have longest chain of five carbons atoms named as pentene.

• We need to number carbon atoms starting from atom nearer to triple bond. Triple bond gets position at $2$nd carbon atom.

Hence, IUPAC name of given compound is $2$-pentene.

We need to give the IUPAC name of below given compound:

• Selecting longest carbon chain containing double bond: So in given structure we have longest chain of four carbons atoms forming a closed ring structure named as cyclobutene.
• We need to number carbon atoms starting from atom nearer to double bond. Now name is 1-cyclobutene.
• We have substituent at $3rd$carbon atom of ring. Substituent is methyl. So name of compound became $3-$methyl cyclobutene.

Hence, IUPAC name of given compound is$3-$methyl cyclobutene.