Which is a relative pronoun and so it can create a new clause. But this is not the split here.

B -- only a few of which are...
C -- but only a few of them are - here but is doing the action of joining clause. But here "but" is better, as there is a contrast.

Refer this : https://www.beatthegmat.com/sub-group-mo ... 92858.html - for "of them" vs "of which"

The distinction between B and C is actually :

The most logical meaning : thousands of salmon swim up to the river in order to spawn.

So : Subject + verb --> thousands of salmon swim
add the adverb of place : salmon swim up to the river
add the adverb of purpose : salmon swim up to the river to spawn.

B has wrong structure :
thousands of salmon spawn by swimming .... - this is incorrect. "by swimming" cannot be the agent here. Moreover, for a passive voice the correct structure is "be + past participle"

A - I am not sure what the prepositional phrase "up the rivers .." is modifying - the action of "swimming" as an adverb or the noun "to spawn".
Consider this :
I wrote the book in our living room.

What is "in our living room" - an adverb of place or an adjective modifying the book. There are many SC on this concept in OG. You can refer some.



Hope this helps !!

I know that 'which' is correctly used after a comma to modify a noun preceding comma OR after a noun + preposition + noun to modify the noun before preposition (or other uses as context suggests).

My question is:
When ( if ever) is 'which' correctly used within a phrase?
Such as, in option B 'which' is used in a phrase 'only a few of
which' rather than right after a comma. When is such a use of 'which' justified?

winniethepooh wrote: My question is:
When ( if ever) is 'which' correctly used within a phrase?
Such as, in option B 'which' is used in a phrase 'only a few of
which' rather than right after a comma. When is such a use of 'which' justified?

I am not able to get you. Can you please elaborate with some examples.

If you question is whether "only a few of which" is right or wrong - then refer my above link. It is correct - "some of which"

Only a few of which -- only a few : noun phrase, "of which ..." : relative clause modifying the noun phrase.

Hope this helps !!

Makes sense! Thanks man!
I Skipped the link while reading your previous post.