Among Li, Na and K, which will turn an aqueous solution of phenolphthalein pink and why?

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Solution

Lithium (Li), Sodium (Na), and Potassium (K) belong to the group $1$ of the periodic table. As we move from top to bottom, the atomic size increases and the ionization energy decreases in a group. So, the basic character of hydroxides increases.
Phenolphthalein is an indicator that changes its colour to pink in an alkaline/basic medium and remains colourless in a neutral and acidic medium.
Metal hydroxides are basic and change colour with Phenolphthalein showing that they are a base.
Li, Na and K form basic hydroxides LiOH, NaOH and KOH, respectively, with water. The strength of these bases is LiOH < NaOH < KOH.
Therefore, among the triad Li, Na and K, all can turn the aqueous solution of Phenolphthalein pink.

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