Which acid is used in non-ionic detergent reactions to improve cleaning qualities and thicken detergents?

The correct option is Stearic acid.

Explanation:

Non-ionic detergents do not contain any ionic groups in their chemical composition. A common method to synthesize non-ionic detergents involves the reaction of a long-chain fatty acid, such as stearic acid (C₁₇H₃₅COOH), with polyethylene glycol (PEG).

The chemical reaction can be represented as:

$C_{17}{H}_{35}\mathrm{COOH}+\mathrm{HO}{\left({\mathrm{CH}}_2{\mathrm{CH}}_{2}O\right)}_n{\mathrm{CH}}_2{\mathrm{CH}}_2\mathrm{OH}\to C_{17}{H}_{35}\mathrm{COO}{\left({\mathrm{CH}}_2{\mathrm{CH}}_{2}O\right)}_n{\mathrm{CH}}_2{\mathrm{CH}}_2\mathrm{OH}+H_{2}O$

Stearic acid acts as a lipophilic (fat-seeking) chain that enhances the emulsifying and cleaning properties of the detergent while contributing to its viscosity and thickening qualities.