And to be even more picky, all three answer options have a mismatch in the parentheses. Here is the corrected version:

-- select 1

SELECT dateadd(day,-(datepart(day,dateadd(mm,1,GETDATE()))),dateadd(mm,-1,GETDATE()))

-- select 2

SELECT dateadd(day,-(datepart(day,dateadd(mm,1,GETDATE()))),GETDATE())

-- select 3

SELECT dateadd(day,-(datepart(day,dateadd(mm,1,GETDATE()))),dateadd(mm,1,GETDATE()))

And for those who prefer to use efficient code to get the last day of the previous month, use either one of these options:

-- Option 1

SELECT dateadd(day, -datepart(day, getdate()), getdate());

-- Option 2

SELECT dateadd(month, datediff(month, getdate(), '20110101'), '20101231');

Option 2 has the additional advantage that you get only the day; all other options include a time part.

(None of this is intended as criticism of a question, just to provide some additional information).