( )Old McDonald has a big farm __ he keeps many animals.(1)which(2)when(3)what(4)where?

(1)which pronoun=which alternate man called "MacDonald"
(4)where=at what place, position=adv.=a big farm.

(4)

應該用關係副詞where，因為He keeps many animals in a big farm.
因為先行詞a big farm是一個地方，所以用where帶出關係子句。

關係詞代名詞which的先行詞通常是事物，關係副詞when是時間。
what是「帶先行詞的關係代名詞」，帶出名詞子句。沒有先行詞。
這三者都不適合本題。