Option Explicit
'в один клик ставим галочку в диапазоне "C5:C23"
'для подсчета проставленных галочек в ячейке С26 формула "=СЧЁТЕСЛИ(C5:C23;"a")"

Private Sub Workbook_SheetSelectionChange(ByVal Sh As Object, ByVal Target As Range)
    Dim i As Integer
    
    If Target.Cells.Count > 1 Then Exit Sub
        If IsNumeric(Sh.Name) Then
            For i = 5 To 26
            If Not Intersect(Target, Sh.Range("C5:C23")) Is Nothing Then
            'If Not Intersect(Target, Sh.Cells(i, 3)) Is Nothing Then
                Target.Font.Name = "Marlett"
                    If Target = vbNullString Then
                            Target = "a"
                        Else
                            Target = vbNullString
                    End If
            End If
                        If Application.WorksheetFunction.CountIf(Cells(i, 3), "a") > 0 Then
                    Cells(i, 2).Interior.ColorIndex = 6
                            Else
                    Cells(i, 2).Interior.ColorIndex = xlNone
                        End If
                     i = i + 1
              Next i
        End If
End Sub