Why is iso propenyl used here or where did that 1 come in methyl ethenyl and where did ethenyl comr from

Dear student

Isopropenyl group is


In the compound given in the question there is a double bond at Carbon -1 and the longest carbon chain is two carbon containing with a methyl group at carbon-1. The IUPAC name of the compound is 1-methyl ethenyl


So, 1-methyl ethnyl group can also be called Isopropenyl

Regards