Final answer:
The element Xx that forms a common strong acid with hydrogen, given the formula HXX, is iodine, represented as I. When iodine and hydrogen combine, they form hydroiodic acid (HI), which is a well-known strong acid.
Step-by-step explanation:
The element Xx that reacts with hydrogen to form a common strong acid with the formula HXX is likely to be a halogen, given that common strong acids include halides like HCl, HBr, and HI. Among the options provided, the correct answer would be element I, which is iodine, since when it reacts with hydrogen, it forms HI (hydroiodic acid), which is known to be a strong acid. Elements like carbon (C) and phosphorus (P) form weak acids (such as H2CO3 and H3PO4 respectively), and francium (Fr) is a highly reactive alkali metal that does not form a binary acid with hydrogen.