Final answer:
Potassium (K) is the most reactive metal among the given options because it is the lowest element in Group 1: The Alkali Metals, and reactivity within this group increases down the group.
Step-by-step explanation:
To determine which of the four elements is the most reactive metal, we need to look at their positions in the periodic table. The elements lithium (Li), sodium (Na), potassium (K), and beryllium (Be) are found in different groups; Li, Na, and K are in Group 1: The Alkali Metals, while Be is in Group 2. The reactivity of alkali metals increases as we move down the group. This is due to the increasing atomic size, which makes it easier for the atoms to lose their outermost electron and thus react. Out of lithium, sodium, and potassium, which are in Group 1, potassium (K) is the lowest in the group, making it the most reactive of the four choices provided.