Isotopes have same:
A. Atomic mass
B. Atomic no.
C. Atomic radius
D. Electronegativity
E. Ionization potential

Hint:
Isotopes have the same no. of electrons and protons but different no. of neutrons.

Complete Step By Step Solution:
First of all you should know what the definition of isotopes is.
Definition of Isotopes: Isotopes are the elements having the same no. of electrons and protons but different no. of neutrons which implies that they have the same mass number.
Now let us check all the properties one by one:

A. Atomic mass: Atomic mass of isotopes is different as their no. of neutrons is different. So this is a wrong option.

B. Atomic no.: Atomic no. of isotopes is the same as atomic no. depends upon no. of protons which is equal in case of isotopes. So this is the correct option.

C. Atomic radius: In isotopes the number of protons and electrons is the same only the number of neutrons changes. So the electromagnetic attraction between the protons and electrons remains the same, so they have atomic radius. So this is also the correct option.

D. Electronegativity: Isotopes have differences in electronegativity but it is really very small. More neutron rich isotopes are less electronegative. It is because in isotopes having more no. of neutrons attraction between nucleus and valence electron is less. So this is an incorrect option.

E. Ionization Potential: Isotopes have the same number of protons and electrons so the electrons present in the outermost shell of these isotopes are the same and the energy required to remove these electrons from the atoms will also be the same which is called ionization energy. So therefore they have the same ionization potential. So this is the correct option.

Note:
Always keep in mind that isotopes have the same no. of protons and different no. of neutrons. So you can find about each property with these two properties. Isotopes are present at the same place in the periodic table.