Answer:
b)Up rose old Barbara Frietchie then,/ Bowed with her fourscore years and ten;
Step-by-step explanation:
a) is not the right answer. This has happened after Barbara leaned out of the window-sill.
b) is the right answer. This line goes before the line in which Barbara leans out of the window-sill (She leaned far out on the window-sill, / And shook it forth with a royal will), therefore this event happened before leaning out.;
c) is not the right answer. Flag-waving at Barbara's grave is an event at the end of the poem, therefore not before she leans through the window.
d) is an incorrect answer. Flag-waving over rebel's heads is an event happening after Barbara's leaning.