。
She bought a loaf of bread.
Literal
She [topic-は] bread [object-を] one-piece bought.
個 (こ) is the most general counter for small objects — apples, eggs, dumplings, packaged items, abstract things. Here it's applied to a portion of bread, but the unit is vague: it could be a single roll, a small loaf, or a packaged unit, depending on context. Japanese bread shops typically sell whole loaves under the counter 斤 (きん) for sliced loaves; 個 here suggests a roll-sized or pre-packaged item. Compare to ピアノを一台 ('one piano') with the counter 台 for furniture and machinery.