You would use the Change event of the parent sheet object - test to see if the cells altered are in Column C and if so what their respective values are - in turn adjusting B as stated.
For a generic and totally untested example:
Private Sub Worksheet_Change(ByVal Target As Range)
Dim rngInterest As Range, rngCell As Range
On Error GoTo ExitPoint
Application.EnableEvents = False
Set rngInterest = Intersect(Target,Columns("C"))
If Not rngInterest Is Nothing Then
For Each rngCell In rngInterest.Cells
If UCase(rngCell.Value) = "X" Then
With rngCell.Offset(,-1)
.Value = LTrim(.Value & " " & .Offset(,-1).Value)
End With
End If
Next rngCell
End If
ExitPoint:
Application.EnableEvents = True
End Sub
to insert the above right click on sheet to which the above is to be applied - select View Code and paste above into resulting window.
Thereafter in a macro enabled file altering value in C to "X" should adjust the string in B such that it appends the present value in A to it.
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