HCl + MnO2 → MnCl2 + H2O + Cl2
In order to balance this, we have to have the same number of electrons of each element on both sides. Here's how to do it:
4HCl + MnO2 → MnCl2 + 2H2O + Cl2
You can see that now there are 4 atoms of H on the left, and 4 on the right (left: 4H, right 2H2 = 4H). The number of electrons for Mn is 1 on both sides, and for O is 2 on both sides. For Cl, on the left, there are 4 electrons, and on the right there are 4 as well (left: 4Cl, right: Cl2 + Cl2 = 4Cl).
So, based on this, the coefficient for HCl is D. 4.
