Question

When forty winters shall besiege thy brow,

Answer 1

Taking into consideration the natural stress that each word has, we can easily identify the stress patter in this line. First, we start with those words which have more than one syllable, and identify the stress pattern within them.

FORty
WINters
beSIEGE

In the context of the sentence, they look like this:

when FORty WINters shall beSIEGE by you.

The words forty winters are the clue here as they establish a pattern of stressed+unstressed that can be used for the rest of the sentence.

when FORty WINters SHALL beSIEGE by YOU.

Therefore, your best answer is option B.