For a standard 45° fillet, the ratio of size of fillet to throat thickness is A. 1 : 1 B. $$1:\sqrt 2 $$ C. $$\sqrt 2 :1$$ D. 2 : 1

The correct answer is $\boxed{\text{B}}$.

A fillet is a rounded corner between two surfaces. The throat thickness is the distance from the fillet to the edge of the part. The ratio of the size of the fillet to the throat thickness is called the fillet radius.

For a standard 45° fillet, the fillet radius is equal to the throat thickness. This is because the fillet is a quarter circle, and the radius of a quarter circle is equal to the diameter of the circle divided by 2.

The other options are incorrect because they do not represent the correct ratio of the size of the fillet to the throat thickness.