What is the formula of the most stable oxide of francium?

Fr2O

Explanation:

Group 1 elements (Li and Na) for oxides by limiting the supply of oxygen during the reaction and super oxides or peroxide in presence of excess oxygen. However in the case of K, Rb and Cs, they form super oxides directly. So it's accurate to say alkali metals form super oxide going down the group with exception of Fr due to it's high polarizability and extremely low charge density, its tends to form oxides similar to sodium and lithium (M2O).

its b it was a compound