Try something like this:
Sub TrimLeadingSpacesColumnD()
Dim ws As Worksheet
Set ws = ActiveSheet
Dim r As Range
Set r = Application.Intersect(ws.Range("D2:D" & ws.Rows.Count), ws.UsedRange)
Dim c As Range
For Each c In r
c.Value = LTrim(c.Value)
Next
r.Select
End Sub
You can use the name of worksheet or Sheets(name) instead of ActiveSheet.