(c) Li
Explanation : The elements with lower nuclear charges are those with lower atomic numbers because the nuclear charge (the positive charge in the nucleus) is determined by the number of protons in the atom's nucleus, which is equal to the atomic number. So, here are the elements in order of increasing nuclear charge :
(a) Beryllium (Be) has an atomic number of 4.
(b) Magnesium (Mg) has an atomic number of 12.
(c) Lithium (Li) has an atomic number of 3.
(d) Potassium (K) has an atomic number of 19.
Among these elements, lithium (Li) has the lowest nuclear charge because it has the lowest atomic number (3), indicating that it has the fewest protons in its nucleus. So, Li has the lowest nuclear charge among the given options.