MC Acid and Alkalis

The answer is B.

A.
Methyl orange changes its colour in slightly acidic solution.
(Colour of methyl orange: pH below 3.1 is red, pH 3.1-4.5 is orange, and pH above 4.5 is yellow.)
When methyl orange is put into dilute HCl (pH ~1), the solution is red. When methyl orange is put into distilled water (pH 7), the solution is yellow. Since the colours are different, methyl orange CAN thus be used to distinguish between dilute HCl and distilled water.

B.
Phenolphthalein changes its colour in slightly alkaline solution.
(Colour of phenolphthalein: pH below 8.3 is colourless, pH 8.3-10 is pink, and pH above 10 is red.)
When phenolphthalein is put into dilute HCl (pH ~ 1), the solution is colourless. When phenolphthalein is put into distilled water (pH 7), the solution is also colourless. Since the colours are the same, phenolphthalein CANNOT be used to distinguish between dilute HCl and distilled water.
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