In this task, you need to answer the given multiple-choice question on geometry. Classify your answers into 'a', 'b', 'c', 'd', and 'e'.
Q: Problem: the area of a circle is increased by 300 % . by what percent has the diameter of the circle increased ? 
Options: ['a ) 400 %', 'b ) 200 %', 'c ) 300 %', 'd ) 100 %', 'e ) 800 %']
A: 
d