Step 1 - Understanding how the radius varies in the periodic table
Sodium and rubidium belong both to IA family. In a family, the radius increases as we go down the family. Therefore, rubidium has a larger radius than sodium.
We may deduce, therefore, how the other periodic properties will vary based on radius difference.
Step 2 - Understanding how periodic properties vary with radius
The smaller the atom, the more difficult it is to remove electrons from it. Therefore, the greater its ionization energy.
The smaller the atom, the stronger the nucleus attracts new electrons. Therefore, the greater its electronegativity.
Step 3 - Answering the exercise
Sodium being smaller than rubidium, it will have a greater ionization energy and will be more electronegative.
Answer: Electronegativity, Ionization energy