Given below are two statements:
Statement I : In the titration between strong acid and weak base methyl orange is suitable as an indicator.
Statement II : For titration of acetic acid with NaOH phenolphthalein is not a suitable indicator.
In the light of the above statements, choose the most appropriate answer from the options given below:

We begin by recalling that in any acid-base titration the chosen indicator must change colour as close as possible to the pH of the equivalence point. This is because, at that pH, the steep vertical rise in the titration curve occurs, and even a single extra drop of titrant causes a sudden jump in pH. If the indicator’s transition range lies completely inside that vertical portion, the end-point and the equivalence point coincide, giving minimum error.

Now, let us examine Statement I: “In the titration between a strong acid and a weak base methyl orange is suitable as an indicator.” A strong acid-weak base titration curve starts at a very low pH (because of the strong acid) and ends at an acidic equivalence point. In fact, the pH at equivalence is usually around $$\text{pH}\approx 3\text{ to }5$$, because the conjugate acid of the weak base hydrolyses and releases $$\mathrm{H^{+}}$$ ions. Hence the vertical segment of the curve lies roughly between $$\text{pH}=3$$ and $$\text{pH}=6$$.

Methyl orange has a colour change interval $$\text{pH}=3.1\;{\text{to}}\;4.4$$. Clearly this entire interval is inside the steep section of the strong-acid-weak-base titration curve. Therefore methyl orange will flip colour exactly where the equivalence occurs, giving an accurate end-point. So Statement I is true.

Next, we analyse Statement II: “For titration of acetic acid with NaOH phenolphthalein is not a suitable indicator.” Acetic acid is a weak acid while sodium hydroxide is a strong base. In a weak acid-strong base titration the pH at equivalence is greater than 7, typically around $$\text{pH}\approx 8.7$$. The vertical portion of this curve generally spans roughly $$\text{pH}=7.5$$ to $$\text{pH}=10$$.

Phenolphthalein changes colour in the interval $$\text{pH}=8.3\;{\text{to}}\;10.0$$. We observe that this entire range lies wholly within the steep rise of the weak-acid-strong-base titration curve, embracing the equivalence pH of $$\approx 8.7$$. Hence phenolphthalein will signal the end-point accurately and is, in fact, the preferred indicator for this titration.

Therefore Statement II is false.

Combining our results, Statement I is true while Statement II is false. The option matching this combination is Option D.

Hence, the correct answer is Option D.