A drop of colourless liquid was placed on blue litmus paper.  The litmus paper turned red.  The liquid could be

In given following options dilute hydrochloric acid is having acidic in nature. So acid can turns blue litmus to red litmus.

Note: Which is releasing ${\mathrm{H}}^{+}$ ions (or) ${\mathrm{H}}_3{\mathrm{O}}^{+}$ ions, in dilution is called “acid”.

Ex: $\mathrm{HC}l+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_3{\mathrm{O}}^{+}+\mathrm{C}{l}^{\mathit{-}}$

$[\mathrm{No}.\mathrm{of} {\mathrm{H}}_3{\mathrm{O}}^{+}=\mathrm{No}.\mathrm{of} {\mathrm{H}}^{+}]$