The LEAST effective form of communication for complex situations is:

the least effective is definitely B.  Since it is a complex situation as you say then written communication will not do at all and might make things worse if it caused misunderstanding which happens a lot in written communication.  A complex situation needs face-to-face communication whether formal or informal that depends on the situation and the involved parties.  But definitely in such a case you need verbal face-to-face communication.  After the face-to-face communication and resolving of matters you can complete it with a written message to wrap up and summarize what has been said.

A and C

Answer B

The least effective form of communication in complex situation is B as what is often written can be misread or misinterpreted. Face-to-face communication can enhance teamwork and achieve greater team building results as body language can be perceived and continuous engagement with team members can be spotted and fostered. Also, clarifications are provided on the spot and misunderstandings dissipated.

E. A and C

A and C

Option E. ( A & C).

E

• Written:  Can be the worst form of communication within an organization as the person who is reading only retains about10% of the total communicated message.  This is not surprising since the reader only has the written word on which to rely.  There is no opportunity to have a2-way conversation, no opportunity for questions and answers, give and take.
• Therefore I go with B

Each one of them  may be the least or the most it's all about your communication style and

the receiver character.

Nevertheless i'll go with the written B 

To the best of my knowledge i will say E. A and C