Both statements are true, and the reason explains precisely why the assertion has to be phrased as an assumption rather than as a fact — option 1.

What the colorimetric method does. An indicator is a weak acid whose acid and base forms differ in colour, and the ratio of the two forms is governed by the pH:

\(pH=pK_{a}+\log_{10}\dfrac{\left[\text{In}^{-}\right]}{\left[\text{HIn}\right]}\)

Matching the observed colour against a chart of standards therefore gives an estimate of pH — and the whole procedure rests on taking equal colour to mean equal pH.

Why that is only an assumption. Colour is not determined by pH alone, and several other influences act on it:

The logical relationship is what makes the answer option 1. Notice that the reason does not contradict the assertion — it accounts for its status. Precisely because colour depends on more than pH, the identity of colour cannot be derived; it has to be assumed, and it holds only when those other factors are matched between sample and standard. The reason therefore tells us why the assertion is an assumption at all, which is exactly what "correct explanation" means here.

What follows in practice. The method is quick and cheap and needs no power, which is why indicator papers remain useful for a rough check. But its accuracy is only about ±0.5 pH, and for anything better the potentiometric method is used instead — a glass electrode developing

\(E=E_{0}-\dfrac{2.303RT}{F}\,pH\)

about 59 mV per pH unit at 25 °C. That method depends on a potential rather than a perception, so it is unaffected by colour and turbidity, and its temperature dependence is explicit in the equation and can be compensated.

Hence, both (A) and (R) are correct and (R) is the correct explanation of (A).