An ionic micelle is formed on the addition of:

A micelle is an aggregate of surfactant molecules in which:
• the long, non-polar hydrocarbon tails crowd together to keep away from a polar solvent, and
• the polar/ionic heads remain in contact with the polar solvent.
This arrangement is possible only when two conditions are met: (i) a surfactant having an ionic (hydrophilic) head and a long hydrophobic tail is present, and (ii) the surrounding medium is highly polar (usually water) so that the ionic heads can stay solvated. A concentration at least equal to the critical micelle concentration (CMC) is also required.

Let us test each option against these conditions.

Option A: “Liquid diethyl ether added to an aqueous NaCl solution.”
No surfactant is introduced—ether is just a neutral, non-polar solvent. Therefore no ionic micelle can form.

Option B: (statement not needed once Option C is confirmed, but any variant given in the exam contains no suitable ionic surfactant in water). Hence no micelle.

Option C: “Sodium stearate added to water.”
Sodium stearate, $$CH_3(CH_2)_{16}COO^- Na^+$$, is an ionic surfactant (soap). In water, when its concentration exceeds the CMC (about $$10^{-4} \text{ mol L}^{-1}$$), the carboxylate heads remain hydrated while the hydrocarbon chains cluster inward, producing normal (direct) ionic micelles. Thus this option satisfies both required conditions.

Option D: “Sodium stearate added to pure toluene.”
Toluene is non-polar, so the ionic heads of sodium stearate would be poorly solvated. Normal ionic micelles (polar outside, non-polar inside) cannot form; at best inverted micelles might appear, which is not what the question asks. Hence this option is incorrect.

Therefore, an ionic micelle is formed only in Option C.

Option C which is: sodium stearate added to water