The most reliable source for a presentation about education in multi-age classes would be a

a school textbook for students.
b parenting blog about student mentors.
c wiki website about the history of one-room schoolhouses.
d journal article about classrooms with students of varying ages.

The answer is D: journal article about classrooms with students of varying ages.

When making an argument, for a presentation or any other form of divulgation platform, it is important to base or found your argument on solid sources that can be corroborated by the audience. The sources of your argument should themselves have been based on research that can be replicated and that, in its turn, has also been corroborated by other researchers (this is the nature of scientific or objective discourse). All the previous characteristics are possessed by journals since their evaluation standards for publications are based on the above mentioned.  

i think that will have something to do with the mississippi river but google will know the full answer for sure