Santa, flying in a sleigh  pulled by reindeer, and going down chimneys, originated from an American poem written in the 1820’s, another American made an engraving called “Santa’s Workshop” in which Santa, is portrayed in a lovely sunset, scratching his chin, reading letters and checking a list of activities, which was revived every year in music and stories told to children during the yuletide (Christmas) season.

解答：

文中有排版错误：a list activities漏掉了介词of,应为a list of activities. activities和which was revived之间应有逗号。

判断定语从句关系代词的先行项，必须结合逻辑和语义。定语从句的意思是每年圣诞季节在音乐中和给孩子们讲的故事中，某个东西又活跃起来（become active）（再次被人们提到、提及。那么什么事情活跃起来，应该是前文提到的“Santa, flying in a sleigh  pulled by reindeer, and going down chimneys, originated from an American poem written in the 1820’s, another American made an engraving called “Santa’s Workshop” in which Santa, is portrayed in a lovely sunset, scratching his chin, reading letters and checking a list activities”整个主句部分。因此，which的先行项是前面整个主句的内容，这就是为什么which后用was。这种以整个主句为先行项的非限制性定语从句前面都必须有逗号。

原文整段内容讲述西方国家有关圣诞老人的习俗的由来。中间部分用一般过去时，讲述一个美国诗人的诗中描述的情景和一个艺术家的雕塑的场面, 这种情景和场面在每年圣诞节期间的音乐中和大人给小孩讲的故事中都再次被提及或讲述，久而久之，就形成了现在每年圣诞节孩子们给圣诞老人写信并要求礼物的习俗。