Mom was busy cooking in the kitchen__________Dad was watching TV comfortably．

句意：当爸爸在舒舒服服地看电视的时候，妈妈正在厨房里忙着做饭。本题考查的是连词的用法。while当…时候，故A正确。
【知识拓展】while当…时候；so因此；since自从；though虽然。
e．g．：(1)They were grinning and watching while one man laughed and poured beer over the head of another．当一个男人大笑着将啤酒倒在另一个男人头上时，他们正边看边咧着嘴笑。
(2)I was tired so I could not help you．我很累因此不能帮你。
(3)I have earned my own living since I was seven，doing all kinds ofjobs．我从7岁起就自己谋生，做各种各样的工作。
(4)I like him．Though he makes me angry sometimes．我喜欢他，尽管他有时惹我生气。