The company is required to acknowledge all complaints in writing within five working days.

acknowledge 是动词，由前缀 ac-（表示"朝向"）和 knowledge（知识、认知）组成，字面意思是"使知晓"。在商业和法律语境中，特指"正式确认收到（某物）"或"承认（事实）"。 在句中用被动语态 is required to acknowledge，表示"被要求确认"。