a. All of the other answers are correct
b. Oxygen-16 and oxygen-18
c. Uranium-235 and uranium-238
d. 238U and 235U
e. A carbon atom with 6 neutrons and a carbon atom with 8 neutrons
Each pairs are from the same elements, meaning they share the same atomic number but different nucleon number (neutron number + atomic number). Thus, all of the answers are correct.
