a$.$ In a priority service, high priority packets are served first. Therefore, packets $2,3,7,8,9,10,$ and $12($which are low priority$)$ will leave the queue after all high priority packets $(1,4,5,6,$ and $11)$ have been served. The slots in which these packets leave the queue would be $6,7,8,9,10,11,$ and $12$ respectively.

b$.$ In a round robin service, packets are served in a cyclic order, regardless of their priority. If packets $1,4,5,6,$ and $11$ belong to one class of traffic and the remaining packets belong to the second class of traffic, the slots in which packets $2$ through $12$ each leave the queue would be $2,4,6,8,10,12,14,16,18,20,$ and $22$ respectively.

c$.$ In a Weighted Fair Queuing $($WFQ$)$ service, packets are served based on their class weights. Class $1$ has a WFQ weight of $1,$ while class $2$ has a WFQ weight of $2.$ This means that for every packet served from class $1,$ two packets will be served from class $2.$ If packets $1,4,5,6,$ and $11$ belong to class $1$ and the remaining packets belong to class $2,$ the slots in which packets $2$ through $12$ each leave the queue would be $3,5,7,9,11,13,15,17,19,21,$ and $23$ respectively.