Which atom has the lowest electronegativity?
This is actually a tricky one.
The answer your teacher is PROBABLY looking for is Fr, Francium
BUT
If you do a deep dive, you'll learn that Caesium (Cs, Cesium) is expected to have a lower electronegativity, due to "relativistic effects". We haven't even actually measured the electronegativity of Francium, and so we're just take a guess, but Cs is expected to be lower. It bucks the trend for ionization energy and is likely to buck the trend for electronegativity as well..