For a solution, the more the $\mathrm{H}^{+}$ ion concentration, solution will be acidic. On the $\mathrm{pH}$ scale, 7 is neutral, higher than 7 is basic and less than 7 is acidic. The $\mathrm{pH}$ scale given below is depicting that as the solution becomes more and more acidic, the $\mathrm{H}^{+}$ion concentration increases and as it becomes more and more basic the $\mathrm{H}^{+}$ion concentration decreases and $\mathrm{OH}^{-}$ion concentration increases. pHSCALEConcentration of $\mathrm{H}^{+} > \mathrm{OH}^{-}$Concentration of $\mathrm{H}^{+} < \mathrm{OH}^{-}$at $\mathrm{pH}=7 (.$ at $25^{\circ}\,\mathrm{C}$ or $77^{\circ}\,\mathrm{F})$ the number of $\mathrm{OH}^{-}$ ions is equal to the number of $\mathrm{H}^{+}$ions. Thus, the correct decreasing order of $\mathrm{H}^{+}$ion concentration in the given acidic sample will be:Gastric Juice $>$ Lemon Juice $>$ dil. Acetic acid $>$ Vinegar