What r isomers and define isomerism​

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In chemistry, isomers are molecules or polyatomic ions with identical molecular formulas — that is, the same number of atoms of each element — but distinct arrangements of atoms in space. Isomerism is the existence or possibility of isomers. Isomers do not necessarily share similar chemical or physical properties.

Mr. Goog.le says

your answer is a. the enthalpy of formation for a pure element in its standard state is always positive.

the awnser is group 12