On Sundays there were a lot of children playing in the park parents sat together joking.

问题补充：

On Sundays there were a lot of children playing in the park, parents sat together joking.

A. their

B. whose

C. which

D. that

答案：

A. their

(答案→)B. whose

C. which

D. that

解析：whose parents sat together joking 为非限制性定语从句，因为句中有完整的谓语 sat。