Just use GROUP BY e.g.
select ID, MAX(A) AS A, MAX(B) AS B, MAX(C) AS C from #t
GROUP BY ID
BUT! As SerenityNowDev has pointed out ... what if you have more than 1 value in A for Id = 1 ... does the
MAX
still give you the result you need or do you need to use another aggregate function? Only you can decide