No need to shout: the answer is A! "Strident," "obstreperous," and "boisterous" all refer to someone or something that is so loud that he, she, or it demands attention. "Strident" focuses mainly on sound and suggests noise that is harsh, discordant, and unpleasant. ("We heard the strident blaring of the tornado warning.") "Obstreperous" is used primarily of people and refers not only to aggressive noisiness, but also to a resistance to being restrained. ("The obstreperous protesters were forced to leave the courtroom.") "Boisterous" is the least disapproving word of the group, referring to noisiness and activity that are due to high spirits. ("The theater was filled with boisterous fans.")