Something like this, I would guess.
create table #testtab (
CId int,
SN char(4)
);
insert into #testtab
select 1, 'ABC1' union all
select 1, 'AAA2' union all
select 1, 'BCV3' union all
select 2, 'XYZ1' union all
select 2, 'MMM1';
select distinct
CId,
STUFF((select ',' + SN from #testtab tt2 where tt2.CId = tt1.CId for xml path('')),1,1,'')
from
#testtab tt1;