( )Don't forget to call your mother __ you arrive in Bali.(1)upon(2)once(3)so(4)with?

(2) once

Don't forget to call your mother upon you arrive in Bali. (X)
Don't forget to call your mother upon your arrival in Bali. (O)
Don't forget to call your mother once you arrive in Bali. (O)

Upon= as soon as 等於 ㄧ⋯⋯就的意思。

但是upon 後面僅接動名詞和名詞 所以答案是3.once