An alkali is titrated against acid with methyl orange as an indicator, which of the following is a correct combination?

First, it is useful to recall the colour behaviour of methyl orange. Experimentally we know that at $$\text{pH}<3.1$$ the indicator appears pinkish-red while at $$\text{pH}>4.4$$ it is yellow. Between these limits the solution shows intermediate orange shades. Hence, whenever the solution moves from an alkaline medium to an acidic one, the visible change is from yellow (basic) to pinkish-red (acidic).

Now we note the typical $$\text{pH}$$ of the equivalence point for the various acid-base combinations:

$$\begin{aligned} \text{Strong acid + strong base}&:&\; \text{pH}\approx7 \\ \text{Strong acid + weak base}&:&\; \text{pH}<7 \\ \text{Weak acid + strong base}&:&\; \text{pH}>7 \end{aligned}$$

Since methyl orange changes colour in the acidic region ($$3.1\!-\!4.4$$), it is suitable only if the equivalence point actually falls within, or at least very close to, this interval. That happens for a titration in which a strong acid is taken against a weak base, because their equivalence point lies distinctly below 7 and usually near 4-5. Conversely, a strong acid-strong base titration gives a neutral $$\text{pH}\approx7$$, which is too high for methyl orange; the indicator would have almost completed its transition well before the true end point, giving a large error.

In the present problem an alkali (i.e. a base) is being titrated against an acid in the presence of methyl orange. To obtain an accurate end point we therefore require the base to be weak and the acid to be strong.

Because the initial solution in the conical flask is basic, its colour at the start is yellow. As the strong acid is run in, $$\text{pH}$$ decreases, and when it falls through the methyl-orange transition range, the colour shifts from yellow to pinkish-red. Thus the observed end-point change must be “yellow → pinkish red.”

Examining the given options, only Option D states “Base: Weak, Acid: Strong, End point: Yellow to pinkish red,” which matches the correct chemical reasoning.

Hence, the correct answer is Option D.