short cut $$2 \times 3 = 6$$
Longer Method
$$\sqrt{6 + \sqrt{6}.....} = x$$
$$6 + x = x^2$$
$$x^2 - x - 6 = 0$$
$$(x - 3)(x + 2) = 0$$
$$x = 3, x = -2$$
x = -2 not possible
So answer is x = 3
Hence the answer is option C
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