last weekend we had a lot of fun ___in the park

c
have fun doing sth

C，have a lot of fun in doing sth.，介词in可以省略，表示“做什么事情有很多乐趣”

C.Playing
a lot of fun 后面要+的是名词，只有C playing是名词，动名词

看句子结构，有主语有谓语有宾语，只能用定语了，选C