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. pHSCALE
Concentration of ${\mathrm{H}}^{+}>{\mathrm{OH}}^{-}$Concentration of 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