What is main function of SOAP message for web services:

a) Web service server configuration
b) XML form of method invocation
c) XML form of signatures of exposed business logic methods
d) Semantic specification of Web services

sounds like a homework question, in which case it's safe to answer a month or so later

Which message do you mean? The request message or the response message?

a) is just a special case of b)
d) is another way of describing c)

b) could be true for the request message, not the response message which would be the result of that method invocation.

c) doesn't have anything to do with the message, but with the WSDL for the service.

So the question cannot be answered using one (or a combination of several) of the supplied options.
b) however comes closest under the restriction that it's valid only under specific conditions for specific types of messages.