The pH range for an acid is pH < 7 with the strength increasing downwards so an acid with pH 1 is much stronger than one with 6.
The range for a Base is pH > 7 with the strength increasing with number thus a substance with pH 14 is much stronger than a substance of pH 8.
With that said, A solution with a pH of 13 would be a STRONG BASE (alkali since it is in solution).
Additional: I attached a pH scale so that at any point in time you can check the pH scale and match it to the substance given and tell whether it is a strong or weak base or acid.