Over a century ago, a prescient educator discovered that children simu

From Meaning perspective B looks good

"multitude of rivers" = subject
"flow" = verb
With the subject and verb, the entire thing is now a clause, but the comparison doesn't make sense:

"children simultaneously process information through multiple sensory channels as a multitude of rivers flow into the ocean" suggests that children process information as rivers flow into the ocean???

But,

"children simultaneously process information through multiple sensory channels as do a multitude of rivers that flow into the ocean."

looks better; however, this sentence (am afraid) suggests that multiple rivers also "process information through multiple sensory channels"

Experts opinion is required.

as a multitude of rivers flowing into the ocean.
Back to the original question:is it a noun or a clause?
this is a noun-phrase (noun + modifier)
multitude of rivers -a noun
flowing- ing participle modifying the noun

we are comparing the flow of information to the flow of rivers
Both involved many sources(structures/rivers) that carry something (information/water) and eventually merged into one large depository(brain/ocean)

We are comparing a clause to a (noun)phrase. This eliminates options with "as"
C,D and E are incorrect because they implied that the rivers also process information.
B-"like" is used to compare a clause to a phrase, and we are comparing similar qualities (the flow of information/river)

IMO B
It's a metaphor, you don't use "as"

Posted from my mobile device

B. Like can be safely used for metaphorical comaprisions such as this one.

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Over a century ago, a prescient educator discovered that children simultaneously process information through multiple sensory channels, as a multitude of rivers flowing into the ocean.

A: as a multitude of rivers flowing into the ocean - as for comparison takes a clause. So it is not correct
B: like a multitude of rivers flowing into the ocean
C: as a multitude of rivers do that flow into the ocean - that can not modify rivers , because of verb 'do' in between. Hence incorrect
D: like a multitude of rivers do that flow into the ocean - Like does not precede a clause. Hence incorrect
E: as do the multitude of rivers that flow into that ocean - Formation is right. But , multitude of rivers do what? process information?Nope. So Incorrect sense conveyed

Left with only B. Correctly compares the multitude of rivers with multiple sensory channels

Just started the day with the comparison. Lets hope everything goes right !

Correct. 'Like' needs to be used in these cases. hence B

Over a century ago, a prescient educator discovered that children simultaneously process information through multiple sensory channels, as a multitude of rivers flowing into the ocean.

A. as a multitude of rivers flowing into the ocean -> we use "as" for verbs and clauses comparison, not "noun phrases". Incorrect.
B. like a multitude of rivers flowing into the ocean -> Correct use of "like". Let's keep it.
C. as a multitude of rivers do that flow into the ocean -> Same as A. Incorrect.
D. like a multitude of rivers do that flow into the ocean -> Noun phrases don't have verbs. Incorrect.
E. as do the multitude of rivers that flow into that ocean -> Same as A. Incorrect.

So, It is B.