Although the bite of brown recluse spiders are rarely fatal, they caus

Just wanted to bring this out that there is nothing wrong with the logical meaning of choice C per se. posing the greatest danger to the infant and elderly is a Participial phrase and is modifying the entire clause it causes chronic flesh wounds. There is nothing wrong with this sense and is perhaps even the preferred sense, since the original sentence is also in this form.

The issue with C is presence of their. their could possibly refer to the infant, the elderly or flesh wounds, but in any of these cases, the meaning is non-sensical.

Although the bite of brown recluse spiders are rarely fatal, they cause chronic flesh wounds, posing the greatest danger to the infant and elderly, who are particularly vulnerable to its poison.
(A) brown recluse spiders are rarely fatal, they cause chronic flesh wounds, posing the greatest danger to the infant and elderly, who are particularly vulnerable to its
(B) brown recluse spiders are rarely fatal, they cause chronic flesh wounds and pose the greatest danger to the infant and elderly, who are particularly vulnerable to their
(C) the brown recluse spider is rarely fatal, it causes chronic flesh wounds, posing the greatest danger to the infant and elderly, who are particularly vulnerable to their
(D) the brown recluse spider is rarely fatal, it causes chronic flesh wounds and poses the greatest danger to infants and the elderly, who are particularly vulnerable to its
(E) the brown recluse spider is rarely fatal, they cause chronic flesh wounds, posing the greatest danger to the infant and elderly, who are particularly vulnerable to its

The bite is the subject of first part of sentence, so...
1> is should be verb to agree with singular subject the bite
2> next sentence should start with singular pronoun it.
Now, Options A, B and E are eliminated.

C is incorrect because 'their' is plural pronoun, whereas sentence talks about singular spider.
Hence Option D is correct choice.

About your doubt:
--> In the second sentence, pronoun should talk about the subject in first sentence. See difference between two lines.
the bite of the brown recluse spider is rarely fatal, it causes ...
The brown recluse spiders bite anywhere on the skin, they cause ...

b/w c and d

if we look at the words in the end..

in c..it says "their" poison
in d..it says "its" poison

n poison should be of the br solders and not of the bite.
"its poison" means posion of the bite but not of the brs..which should be the other way round

kindly help me on this.
so why not c

With C, the sentence would be:

Although the bite of the brown recluse spider is rarely fatal, it causes chronic flesh wounds, posing the greatest danger to the infant and elderly, who are particularly vulnerable to their poison.

their, a plural pronoun, cannot refer to either spider or bite, both of which are singular.