What type of channel do speaker 1 and speaker 2 use in their communication?
a. visual
b. auditory
c. olfactory
d. tactile

The type of channel that speaker one and speaker two use in their communication would be auditory - b.

The fact that we're talking about "speaker" 1 and "speaker" 2 implies already the fact that both of them are talking and this then implies that they are using the auditory channel of communication. 

The correct answer is "  it was believed that the dispute emerged in 632 just after the death of the prophet muhammad."