The amu in each of the answers, stands for mass of the element. This is a combination of both the proton and neutron mass. From you options, oxygen is the element with highest mass.
The element that has the highest combined number of protons and neutrons is Oxygen: 16 amu.
The atomic mass unit (amu) of an element is the sum of the number of protons and neutrons in the atom of that element.
The atomic mass unit of the element reveals the total number of protons and neutrons in the element.
Since oxygen (16 a.m.u) has the highest atomic mass unit among the given elements in this question, therefore, it has the highest combined number of protons and neutrons.
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