Final answer:
Cesium (Cs) is the most reactive element among the given options, as it is part of the highly reactive alkali metals group, which includes Li, Na, K, Rb, and Fr.
Step-by-step explanation:
The element from the options given (Li, An, Rn, Cs) that is the most reactive is cesium (Cs). Cesium is a member of the alkali metals group in the periodic table, which also includes lithium (Li), sodium (Na), potassium (K), rubidium (Rb), and francium (Fr). As you move down the group in the periodic table, the reactivity of the alkali metals increases due to the increasing atomic size and decreasing ionization energy. Although not explicitly listed in your options, 'An' likely refers to an unknown element and 'Rn' to radon, which is a noble gas and not very reactive. Therefore, cesium is the correct answer because it is the most reactive among the listed elements.