declare @t table(Id int, ColA int, ColB int, Isdefault bit);
insert into @t values
(1, 100, 96, 1),
(2, 100, 96, 0),
(3, 101, 98, 0),
(4, 102, 99, 1);

with cte as
(
select *, row_number() over(partition by ColA, ColB order by Isdefault desc) rn
from @t
)

select Id, ColA, ColB, Isdefault
from cte
where rn = 1;