The problem is too complex. We ought to ___ a specialist.

我觉的应该是D
Call in 作"请来"、"找来"、"召来"解，"请人来作某种专业的咨询或帮忙"的意思。如：
Your father is ill, you should call in a doctor at once.你父亲病了，应该马上去请个大夫来。
The police have been called in to help make it clear.
已请来了警察帮忙把这件事弄清楚。

Call on和call at的意思一样，都是"拜访""访问"，但call on后接表示人的名词或代词，call at 后接表示地点或场所的词语。试比较：
I called on the Smiths yesterday.
我昨天拜访了史密斯一家人。
I called at the Smith's yesterday.
我昨天去了史密斯家。
I called on the doctor yesterday.
我昨天去拜访了医生。
I called at the doctor's yesterday.
我昨天去了医务室。
另外，call on还可以"号召"，后面往往接动词不定式作宾语补主语。如：
The headmaster called on the students to work harder.校长号召学生们更努力地学习。

Call for 有几个不同的意思：
1．作"要求"、"需要"、"提倡"解。如：
Success calls for hard work.
成功要靠勤奋。
People are calling for the freedom of all slaves.人们要求解放所有奴隶。
The work calls for a lot of time.
这项工