Carl was calmly taking a photograph as this ____________ large bear walked towards the car.

In this sentence, the correct form of the word is "astounding," which functions as an adjective describing the bear. The phrasing suggests that the bear is surprising or impressive in size as it approaches the car.

"Astounding" is the appropriate choice here because it conveys the sense of something that causes great surprise or wonder. It describes the bear's size and presence in a way that amplifies the dramatic setting.

The other choices do not fit the context: "astonishing" could have been a close contender, but it is somewhat less suited in this particular context. "Astonishingly" is an adverb and does not properly modify the noun "bear" in this structure. "Astound" is a verb, which also does not align with the need for a descriptive adjective in this sentence.

Thus, "astounding" is the only option that correctly enhances the meaning and grammatical structure of the sentence.