Which one of the following can be used as an acid-base indicator by a visually impaired (blind) student?

As per the NCERT text (Section 2.1.1, Activity 2.2), besides the usual acid-base indicators that work by a colour change, there are olfactory indicators — substances whose odour changes in acidic or basic media. The chapter explicitly tests onion, vanilla essence and clove oil for this property, and notes that on treating dilute vanilla essence with dilute HCl and dilute NaOH separately, its characteristic smell changes/disappears in one of the media, so the change (or loss) of smell can be used to identify acids and bases without seeing a colour. This makes it usable by a visually impaired student.

• (A) Turmeric — it is a natural indicator, but it works by turning red/reddish-brown in a base (as with the curry stain example in the chapter's introduction) and stays yellow otherwise — this is a colour change, which a blind student cannot perceive.
• (C) Methyl orange and (D) Litmus are also indicators mentioned in Table 2.1 of the chapter, but both act only through a change of colour (litmus: red↔blue; methyl orange: changes colour in acid/base), which again cannot be used by a visually impaired student.

Hence only vanilla essence, an olfactory indicator, can be used by touch/smell-based testing, making option (B) correct.