Boys and girls,please don't talk so＿＿,for there is someone sleeping in.

C.loudly 因为它修饰talk，talk是动词，动词用副词来修饰。

分析：C
loud 1、adj.(-er, -est) 喧闹的; 响亮的: loud voices, screams, laughs, etc 宏亮的嗓音、 喊声、 笑声等 * That music's too loud; please turn it down. 那音乐太吵人了; 请把音量调低些.
2、adv.(-er, -est) 尤与talk、 sing、 laugh等动词连用: laugh loud and long 大笑不已 * Speak louder I can't hear you. 大点声说--我听不见.

aloud
adv.出声地（用足以使人听得到的声音, 既非默不作声地亦非悄悄声地）: He read his sister's letter aloud. 他喃喃地读他妹妹的信.
大声地, 高声地（使得在远处也听得见）: She called aloud for help. 她高声呼救

loudly
adv.高声地；响亮地She screamed as loudly as she could.

so后面应为副词（修饰talk），又从意思上理解，正确选项为C

C,
用副词修饰动词、、

C