There isn't a direct data validation option for case sensitivity in Excel. However, you can achieve this using formulas within the data validation criteria. Here's one approach:

1. Using the EXACT Function and Data Validation:

* Prepare your list (Optional): If you have a specific list of allowed values (uppercase or lowercase), create a list of these values in a separate area of your worksheet.
* Select the cell(s) for validation: Choose the cell(s) where you want to enforce case-sensitive data entry.
* Go to Data Validation: Navigate to the "Data" tab and click "Data Validation" in the "Data Tools" group.
* Set Allow to Formula: In the "Settings" tab, under "Allow," choose "Formula" from the dropdown menu.
* Enter the Formula: In the "Formula" box, type the following formula, replacing `A1` with the cell reference where you want to validate the data:

```excel
=EXACT(A1,UPPER(A1)) // For uppercase validation
=EXACT(A1,LOWER(A1)) // For lowercase validation
```

Explanation:

- `EXACT(A1, UPPER(A1))`: This formula checks if the value entered in cell A1 (`A1`) exactly matches the uppercase version of itself (`UPPER(A1)`).
- `EXACT(A1, LOWER(A1))`: This formula checks if the value entered in cell A1 (`A1`) exactly matches the lowercase version of itself (`LOWER(A1)`).

* Error Message (Optional): You can set an informative error message to display if the user enters a value that doesn't meet the case sensitivity criteria. In the "Input Message" box, type a clear message like "Please enter text in uppercase/lowercase only."

* Click OK: Click "OK" to save the data validation settings.

Now, when a user enters data in the specified cell, Excel will only accept values that match the exact case (uppercase or lowercase) defined in the formula. Any other case will trigger the error message (if set).

Note:

- This approach validates against the entered value's case, not the existing data in the cell.
- If you have a pre-existing list of allowed values, you can replace `UPPER(A1)` or `LOWER(A1)` with a reference to your list containing the uppercase or lowercase versions of the allowed values.