Which of the Following Is the Correct Definition of an Isomer

Which of the following is the correct definition of an isomer?

A) Isomers are molecules that have the same empirical formula but different arrangements of atoms in space.
B) Isomers are molecules that have the same molecular formula but different arrangements of atoms in space.
C) Isomers are molecules that have the same arrangements of atoms in space but different empirical and molecular formulas.
D) Isomers are molecules that have the same molecular formula but contain different isotopes.
E) Isomers are molecules with the same empirical formula but have different isotopes.