The principal quantum number $n$ can take values
A$1, 2, 3, \ldots$
B$\pm 1/2, \pm 3/2, \ldots$
C$0, \pm 1, \pm 2, \ldots$
D$0, 1, 2, \ldots$
Answer & Solution
Correct answer: A. $1, 2, 3, \ldots$
$n$ is a positive integer — it labels the shell ($K, L, M, N, \ldots$). $n = 0$ is forbidden because it would mean zero radius.
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