How to retrieve multiple rows into single row

create table StudentMark (REG_NO int,REF_NO int,SUB varchar(3),MARKS int)

insert into StudentMark values 
(16015090,015047,       'GSC',  9),
(16015090,015047,	'SSC',	28),
(16015090,015047,	'MTH',	21),
(16015090,015047,	'TLS',	4),
(16015090,015047,	'FLO',	44),
(16015090,015047,	'SLE',	2),
(16033545,033114,	'MTH',	13),
(16033545,033114,	'TLH',	39),
(16033545,033114,	'FLO',	12),
(16033545,033114,	'SLE',	13),
(16033545,033114,	'GSC',	5),
(16033545,033114,	'SSC',	11),
(16056438,056007,	'FLO',	19),
(16056438,056007,	'SLE',	10),
(16056438,056007,	'MTH',	25),
(16056438,056007,	'TLS',	21),
(16056438,056007,	'GSC',	23),
(16056438,056007,	'SSC',	17)

select * from StudentMark

select ROW_NUMBER()over(partition by REG_NO order by SUB)Slno,* into #prps from StudentMark

select distinct REG_NO,REF_NO into #detail from StudentMark

DECLARE @slno int
select @slno =max(slno) from #prps
DECLARE @i varchar(10),@alter varchar(4000),@value varchar(4000)
SET @i=1
while(@i<=@slno)
begin
select @alter='alter table #detail add SUB'+@i+' varchar(100),Marks'+@i+' varchar(100)'
select @value='update #detail set SUB'+@i+'=b.sub,Marks'+@i+'=b.Marks from #detail a join #prps b on a.REG_NO=b.REG_NO where slno='+@i
exec (@alter)
exec (@value)
set @i=@i+1
end

select * from #detail

REG_NO	REF_NO	SUB1	Marks1	SUB2	Marks2	SUB3	Marks3	SUB4	Marks4	SUB5	Marks5	SUB6	Marks6
16015090	15047	FLO	44	GSC	9	MTH	21	SLE	2	SSC	28	TLS	4
16033545	33114	FLO	12	GSC	5	MTH	13	SLE	13	SSC	11	TLH	39
16056438	56007	FLO	19	GSC	23	MTH	25	SLE	10	SSC	17	TLS	21