The correct statement about isomers is:
Enantiomers differ in biological activity.
Enantiomers are a type of stereoisomer that are mirror images of each other and can have different effects in biological systems. For example, one enantiomer of a drug may be therapeutically active, while the other is inactive or may even have harmful effects.
The other statements are incorrect:
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Cis-trans isomers are mirror image isomers. (This is false; cis-trans isomers (or geometric isomers) refer to the orientation of groups around a double bond or a ring, not to mirror images. Mirror image isomers are enantiomers.)
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Structural isomers differ in the position of double bonds within the molecule. (This statement is misleading. Structural isomers can differ in various ways, including the position of double bonds, but it isn’t accurate to define structural isomers solely by the position of double bonds.)