Here's why:

* Valence Electrons: These are the electrons in the outermost shell of an atom. They are the ones involved in chemical bonding.

* Periodic Trends: Elements in the same group (vertical column) of the periodic table have the same number of valence electrons. This is why elements in the same group share similar chemical properties.

* Lithium and Francium: Both are in Group 1 (alkali metals) and have one valence electron.

Let's look at the other options:

* The same number of electron shells: While both lithium and francium are in the same group, they have different numbers of electron shells. Lithium has 2 shells, while francium has 7.

* Of? This option is incomplete and doesn't provide a relevant reason for their similarities.

Therefore, the correct answer is B.