In the beach house,there was a small room（）A.being served as

there was a small room （served as a kitchen ）做定语修饰room ,房间和serve（当做.使用,用做）之间是被动关系,所以选B
D选项 in which=where 相当于是副词做状语,
若选d in which served as a kitchen
——— ———— ————
状语 谓语 宾语 句子结构不完整,故不能选
再问： 为什么说是副词做状语！？
再答： 副词一般在句子中做状语．而且这道题如果选d的话，应该是把它考虑成定语从句了，在定语从句中 in which=where ，where 为关系副词后接主谓宾齐全的句子 例如：this is the factory where i worked. this is the factory which i saw yesterday