Purity of a solid organic compound is classically assessed by its melting point behavior.

• A pure crystalline solid melts sharply over a narrow range (often ≤1–2 °C).
• Impurities disrupt the crystal lattice, causing the melting point to be lower and the melting range to broaden.

So simply measuring the melting point (and comparing the sharpness/range to literature values for the pure acid) quickly reveals whether the isolated solid is pure or contaminated.

The other techniques listed give information about identity or quantity, not purity:

• Titration with NaOH tells you the equivalent weight or moles of acidic protons present, which helps determine molar mass/identity, but a contaminated sample could still titrate cleanly if the impurity is non-acidic — it doesn't reveal purity.
• Combustion analysis gives the empirical formula (mass % C, H, O), useful for identifying the compound, not for detecting small amounts of impurity.
• UV-Vis spectrophotometry is useful for compounds with chromophores and for quantitative concentration measurements, but many carboxylic acids lack strong UV-Vis absorption and this method isn't a standard purity check for isolated solids.

Hence, melting point determination is the most direct and useful purity assessment.