The most suitable indicator to be used when reacting ethanedioic acid with potassium hydroxide is

Choosing the right indicator for a titration depends on the strength of the acid and the base being used, because different indicators change colour at different pH ranges.

Ethanedioic acid (oxalic acid, HOOC-COOH) is a weak diprotic acid, and potassium hydroxide (KOH) is a strong base.

In a weak acid vs. strong base titration, the equivalence point lies in the basic/alkaline region (pH roughly 8-10). This is because at the equivalence point, the solution contains the conjugate base of the weak acid (potassium ethanedioate/oxalate), which hydrolyses to make the solution slightly basic.

Phenolphthalein changes colour in the pH range of approximately 8.2 to 10.0 (colourless in acid, pink in alkali). This range coincides with the equivalence point of a weak acid-strong base titration, making it the most suitable indicator.

Methyl orange (pH 3.1-4.4) and methyl red (pH 4.4-6.2) change colour in acidic pH ranges and would signal the endpoint too early, before the true equivalence point is reached. Methyl purple is less commonly used and is not the standard choice for this type of titration.