He always stammers in public meetings, but his today’s speech . . . . . . . . . .

Defining "stammer":
To stammer means to speak with sudden involuntary stops and repetitions.
Analyzing the Options:
Option A: was fairly audible to everyone. This is plausible, as audibility doesn't directly contradict stammering. He could have stammered slightly but still been heard.
Option B: was not liked by the audience. This doesn't directly address the contrast introduced by his usual stammering. The audience's opinion is unrelated to his fluency.
Option C: as not received by the audience. This is grammatically incorrect and doesn't make sense in context.
Option D: was surprisingly fluent. This option is the best fit. "Surprisingly fluent" directly contrasts with his usual stammering. It shows that something unexpected and positive happened in his speech delivery.
Correct Answer Explanation:
The sentence requires an answer that highlights a positive change in his speech, contrasting his usual stammering. Option D, "was surprisingly fluent," perfectly captures this contrast. The word "surprisingly" emphasizes the unexpected nature of his fluent speech, making it the most suitable and grammatically correct choice.