选:C.is
who 引导定语从句,修饰the only one of the staff members
当先行词为 the only one of +名复数时,定从的谓动用单数,
又:D中的现在完成时态不适合本句,因此答案选C.
be to do sth.即将做某事
句意:汤姆是即将晋升的唯一职员.