【正确答案】 B

【答案解析】[解析] 题意：他们把空闲的卧室改成了一间办公室。
划线词convert(vt.)的词义是“转换．改造”。
A项reduce意为减少；例句：They are selling the goods under the counter at reduced prices．他们正在削价偷偷出售商品。
B项turn意为改变，使改成。例句：Nothing will ever turn him from his purpose．什么也不能使他改变目标。
C项move意为移动；例句：The army is on the move．部队在行军。
D项reform意为改革；例句：Because of his encouragement, she won the best designer by her reformed cheongsam．因为他的鼓励，她以她的改良式旗袍赢得了最佳设计师奖。
很明显B项正确。