In "Dulce et Decorum Est," what is the rhyme scheme of stanza two?
Whenever we think of rhyme schemes in a poem we normally map them by matching a letter to particular rhyme for the duration of the poem and seeing how it continues. Therefore to give you a simple example, a poem which said "There was a boy called Jack / Who lived in a great big shack" would have the rhyme scheme of AA because of the same rhyme in "Jack" and "shack." At every different rhyme, a new letter is added.
So, thinking about the second stanza, there is a definite rhyme in "fumbling," "stumbling," and "drowning" and likewise a different rhyme is present in "time" and "lime." The penultimate line is unique in not having a rhyme that matches it anywhere else. Thus the rhyme scheme of the second stanza can be described as: ABABCA. A matches the "-ing" words and B matches the "-ime" words. C of course stands for the line ending with "light."