回答：此处应该用that,原句应该是：They had their supper at five o’clock in the afternoon.
强调句型it was ...that 强调了“at five o’clock in the afternoon” .
欢迎提问,这不是强调句不能用that继续回答：不是强调句型是什么句型呢？如果句子是这样的，即，去掉at,就不是强调句型：It was five o’clock in the afternoon ( when) they had their supper .欢迎追问，乐意解答；愿你满意, 望你采纳。不是at是about，打错继续回答：如果是about，就不是强调句型，而是定语从句，就不能用that，要用when。 欢迎追问，乐意解答；愿你满意, 望你采纳。