They tolerated him though they disliked his craven behaviour.

The correct answer is B. cowardly.

Craven means “showing or feeling a lack of courage.” In the context of the sentence, “They tolerated him though they disliked his craven behaviour,” the speaker is saying that they put up with the person’s behavior even though

The other options are incorrect because they do not accurately describe the person’s behavior.

• Silly means “not showing or using good judgment.” The person’s behavior may have been silly, but the speaker is not saying that they thought it was silly.
• Indecent
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  means “not decent or proper.” The person’s behavior may have been indecent, but the speaker is not saying that they thought it was indecent.
• Mean means “unkind or spiteful.” The person’s behavior may have been mean, but the speaker is not saying that they thought it was mean.

In conclusion, the correct answer is B. cowardly.