Answer:
Hydrochloric acid - HCl.
Step-by-step explanation:
Hello,
In this case, by rewriting the undergoing chemical reaction we've got:

For determining the limiting reagent, one compares the moles of manganese dioxide with those of the hydrochloric acid, for instance, by finding the moles of the MnO₂ that completely react with 10.0 moles of HCl as given on the statement as shown below:

In such a way, since 2.5 moles of MnO₂ would be consumed but 3.0 moles are available, there will be 0.5 unreacted moles of it, therefore, the limiting reagent is the hydrochloric acid, HCl as it will be firstly consumed.
Best regards.