Lock in dropdown selection - excel

Question

I'm creating a quiz.
Questions on one worksheet and the answers on another.
When a question is answered another field with this formula
=IF(C5="","",IF(C5=Answers!A5,"Correct","Incorrect"))
tells the person if the answer is correct or incorrect.
I am using data validation with dropdown lists so they can only choose true/false, (a, b, c, d) etc.
Is there a way to lock in a selected answer, until a master reset button is pressed?
For example,
Question is in A1
The possible answers are in the form of a dropdown menu in B1.
Sometimes the answer is in the form of a true false, sometime it is in the form of a multiple choice. In the example of true false, if the person puts in true, c3 will say correct or if they put false, then incorrect.
As it is now, the person can switch back and forth as much as they want.
I am looking to make it so once an answer is selected, they cannot change it.

Answer 1

You can use Sheet Protection, combined with Range Locking and a Change event.
Put this code in the relevant Worksheet Module. Adjust the Private constants to suit your needs.
Option Explicit
' Reference the cells that your users may enter data into
Private Const DataCells As String = "J1,J3,J5"
Private Const PW As String = "password"
Private Sub Worksheet_Change(ByVal Target As Range)
Dim cl As Range
Dim DataRange As Range
Set DataRange = Me.Range(DataCells)
'Loop thru changed cells
For Each cl In Target.Cells
'If changed cell is in the DataCells range and is not blank, lock it
If Not Application.Intersect(cl, DataRange) Is Nothing Then
If Not IsEmpty(cl) Then
Me.Unprotect PW
Target.Locked = True
Me.Protect PW
End If
End If
Next
End Sub
'Re-enable data entry to all DataCells
Sub MasterReset()
'Unlock the sheet, prompt for password
Me.Unprotect
'Unlock the cells
Me.Range(DataCells).Locked = False
'Optional, clear DataCells
Me.Range(DataCells).ClearContents
'Lock the sheet again
Me.Protect PW
End Sub

Answer 2

This works pretty good:
In the "This Workbook" module, insert the code:
Private Sub Workbook_Open()
Sheet1.Protect userinterfaceonly:=True 'allows macros to run
Sheet1.Range("A1:A20").Locked = False 'replace this range with the range the user deals with.
End Sub
In the sheet module that the user will be interacting with, add this code:
Private Sub Worksheet_SelectionChange(ByVal Target As Range)
If Target.Locked = True Then Exit Sub
If Target.Locked = False Then
If Target.Value = "" Then Exit Sub
If Target.Value <> "" Then Target.Locked = True
End If
End Sub
That should take care of things for you!