Reactivity in metals generally increases as you move down a group in the periodic table and decreases as you move from left to right across a period.
In the case of the metals mentioned:
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Lithium (Li) is in Group 1 (alkali metals) and is quite reactive.
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Sodium (Na) is also in Group 1 and is more reactive than lithium.
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Potassium (K), being below sodium, is even more reactive.
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Beryllium (Be) is in Group 2 (alkaline earth metals) and is less reactive than lithium.
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Magnesium (Mg) is also in Group 2 and is more reactive than beryllium but less reactive than sodium and potassium.
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Calcium (Ca) is below magnesium and is more reactive than both lithium and beryllium.
Comparing the reactivity of these elements, Beryllium (Be) is less reactive than the others listed (Lithium, Magnesium, and Calcium).
Thus, the answer is Beryllium (Be).