The correct answer is B. it does not follow a formal structure.
Let us first go through all of these options in order to determine the structural characteristics of <em>Forgetfulness, </em>by Billy Collins.
Option A is incorrect - if you count the syllables in each line, you will see that the first one has 11, the second one 13, and so on - so, not 10.
Option C is also incorrect - there are actually 8 stanzas, and they are not of equal length.
Option D is also incorrect - there are no rhymes in this poem.
So, by the process of elimination, the correct answer is B - this poem doesn't follow a formal structure - it is rather a free verse poem containing stanzas of varying lengths.