Hi sukkhi,

I agree with you, B sounds good.

"Becoming" in sentence B is a linking verb (sometimes called a copular verb). Linking verbs show a relationship between the subject (species X, in your example) and the sentence complement, the part of the sentence following the verb (extinction, in your example). Linking verbs sort of act like an equal sign in the sentence, because they provide the connection between a subject and a certain state. The most common linking verbs are forms of "to be," "to become" and "to seem."

I hope this helps!

thanxx david

can u tell me any error in (a)

Hi sukkhi,

The noun preposition noun construction ("danger of extinction") is awkward, but it is probably only technically incorrect if "danger of extinction" is treated as a really big noun, like "House of Horror" or "Temple of Doom." In sentence A I think you can argue that it is NOT treated in this manner, hence B is a better way to express the idea.

PREPOSITION + VERBing:
In this construction, the VERBing functions as a NOUN.
To be more specific, the VERBing serves as the OBJECT of the preceding preposition.
A VERBing that functions as a noun is called a gerund.

SC2 in the OG12: a large amount of SPENDING
Here, spending is a gerund serving as as the object of the preposition of.
SC10 in the OG12: that keeps the brain from GETTING
Here, getting is a gerund serving as the object of the preposition from.
SC33 in the OG12: will cut production by CLOSING
Here, closing is a gerund serving as the object of the preposition by.

As you can see, PREPOSITION + VERBing is a common construction.

(a)- species x is in danger of extinction.
(b)- species x is in danger of becoming extinct

In B, becoming is a gerund serving as the object of the preposition of.
Both A and B are correct.
Option A might be preferable because it a bit more concise.
Generally:
If an SC offers a choice between an actual noun (such as extinction) and a VERBing (such as becoming extinct), the actual noun will be preferable.