Given below are four sentences, out of which one sentence is grammatically correct. Choose the option which points out the grammatically correct sentence. If all sentences are incorrect. Choose ‘All sentences are incorrect’ as your answer.

Correct option is B

Below is an evaluation of the correctness of all the sentence in the options.
In statement (a), the error lies in the use of the word "exceeding" instead of "exceedingly." "Exceeding" is not the appropriate form to describe the degree of monotonousness. Therefore, the correction involves replacing "exceeding" with "exceedingly." The corrected sentence reads: "The advertisement was exceedingly monotonous, repeatedly restating the same thing within a thirty-second time frame."
In statement (b), there is no grammatical error. The sentence is correct as it is. The subject-verb agreement and verb tense are accurate. The correction is not required.
In statement (c), the error in the sentence is the use of "of" after "despite." The word "despite" does not require the preposition "of" in this context. Here's the corrected sentence:
"Despite the heavy rain, we decided to go for a walk in the park."
In this corrected version, "despite" is used correctly as a preposition, followed directly by the noun phrase "the heavy rain." The phrase "despite of" is incorrect in standard English.
In statement (d), the error lies in the subject-verb agreement. The phrase "The group of activists" is singular, so the verb "have" should be changed to "has" to match the singular subject. Therefore, the corrected sentence is: "The group of activists has launched yet another campaign discouraging medical tests on animals."