Answer: Choice A.

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Step-by-step explanation:
Recall that 36 is a perfect square
36 = 6^2
Or in other words,

The point marked on this number line is some value smaller than 6, so the number under the square root must be smaller than 36.
That could apply to either choices A or B.
Using a calculator shows that

Choice A is the better answer as it's much closer to 6 than choice B is.