She___when she began to talk about her mother.

choked up (因激动)说不出话来
choked off 劝阻;阻止
这句话的意思是当她开始和她母亲说话时，她激动地说不出话。所以选A

choked up 为一固定搭配

A choked up是过分激动的意思
而B choked off是中断的意思
如果用B的话,整个句子的意思不明确.通顺
所以用A