The correct answer is Option C: Phenolphthalein, colourless to pink. Here's why:

Titration of Sodium Hydroxide (NaOH) against Oxalic Acid (H₂C₂O₄)

Sodium hydroxide is a strong base, and oxalic acid is a weak acid. The reaction between them is a neutralization reaction:

$$2NaOH + H_2C_2O_4 \longrightarrow Na_2C_2O_4 + 2H_2O$$

Choosing the Right Indicator

An indicator is a substance that changes color at a specific pH range, signaling the endpoint of the titration. The ideal indicator for a titration should have a color change near the equivalence point of the reaction. The equivalence point is the point where the moles of acid and base are stoichiometrically equal.

In this case:

• At the equivalence point, the solution will be slightly basic due to the formation of the sodium oxalate salt (Na₂C₂O₄), which is the conjugate base of a weak acid.
• Phenolphthalein changes color in the slightly basic pH range (around 8.2 to 10.0), making it an appropriate indicator for this titration.

Color Change

Phenolphthalein is colorless in acidic solutions and turns pink in basic solutions. So, the color change at the endpoint of this titration will be:

Colorless (before equivalence point) → Pink (at equivalence point)

Why Other Options Are Incorrect

• Option A: Phenolphthalein does not change from pink to yellow, it changes from colorless to pink.
• Option B: Alkaline KMnO₄ is not a suitable indicator for this titration because it is not sensitive enough to detect the slight pH change at the equivalence point.
• Option D: Methyl orange changes color in the acidic pH range (around 3.1 to 4.4) and is therefore not suitable for this titration.