Answer & Solution
Let's understand the other options:
Option A, "opaque()", is not a valid function used with the filter property. It does not apply blurring effects.
Option B, "scatter()", is also not a valid function used with the filter property. It does not apply blurring effects either.
Option D, "all of the mentioned," is incorrect because options A and B are not valid functions used for blurring images with the filter property.
Therefore, the correct answer is option C, "blur()", as it is the function used by the filter property to blur images.