In a hypothetical universe, if atomic numbers were used to form triads, which of the following will make triad with Na and K:
P
In this hypothetical universe, we have to use atomic numbers to construct triads.
Thus, Na with atomic number 11 and Potassium with atomic number 19 are at the ends of the triad.
Their average, i.e. 15 should be the atomic number of the middle element. Potassium (atomic number = 15) is the middle element.