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Answer
Correct option: D.sometimes a natural number and sometimes an irrational number.
$a.$Is incorrect, because $\sqrt{\text{n}}$ can not be always a natural number
i.e. if $\text{n}=2, \ \sqrt{\text{n}}=\sqrt{2} ($not a natural no$.)$
$b.$Is incorrect, similiarly, if $n = 2, 5, …. $ Or any odd no. or not perfect square, $\sqrt{\text{n}}=\sqrt{2},\sqrt{5},\sqrt{7}$ are Non$-$terminating and non$-$repeating,
So irrational in nature, So, not always a rational number.
$c.$Is also incorrect, $\sqrt{\text{n}}$ can aslo be rational or say a natural number.
If $n = 4, 9, 16...$ or any perfect square number then $\sqrt{\text{n}}=2,3,4...$ natural numbers.
$d.$Is fully correct because if $n$ is any odd number or non$-$perfect square number then $\sqrt{\text{n}}$ would be irrational, but if $n$ is a perfect square number $\sqrt{\text{n}}$ then will be a natural number.
If $n = 2, 3, 5, 8 ... \sqrt{\text{n}}=\sqrt{2},\sqrt{3},\sqrt{8}... ($irrational$)$
If $n = 4, 9, 16 ... = 2, 3, 4 ... ($Natural number$)$
So, correct option is $(d).$
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