The correct option is A to stand erect at all times
These lines:
"Though storms may flatten you.
Weep not at fate,
But spring erect
As bamboo thickets do"
mean that when you (friend) are overwhelmed with troubles, do not cry over your fate, but spring back up (stand erect) like bamboo thickets stand.
Option D contains a similar line, hence completing the sentence: The poet asks her friend to stand erect at all times. Hence D is the correct option.
Option A: This is the opposite of what the poet asks her friend to do: "weep not at fate" (do not weep at fate). Hence A is incorrect.
Options B and C: Similarly, these lines are the opposite of what the poet asks her friend to do. Hence B and C are also incorrect.