Explanation:

In MS Excel, the Fill Series dialog box allows users to automatically fill a series of cells with values that follow a specific pattern. When using the Fill Series dialog box, the "Step Value" option allows users to specify the difference between each value in the series.

The default value for the "Step" in the Fill Series dialog box is 1, which means that the series will increase or decrease by 1 for each subsequent cell.

Here's an explanation of each option:

- Option a) 0: This option does not make sense as a step value since it would result in a static series with no change in value.

- Option b) 1: This is the correct answer. The default step value in MS Excel's Fill Series dialog box is 1. This means that if a user fills a series starting from a specific value, each subsequent cell will contain a value that is incremented or decremented by 1.

- Option c) 5: This option is not the default step value in MS Excel's Fill Series dialog box. However, users can manually set the step value to 5 if they want the series to increase or decrease by 5 for each subsequent cell.

- Option d) 10: This option is also not the default step value in MS Excel's Fill Series dialog box. Users can manually set the step value to 10 if they want the series to increase or decrease by 10 for each subsequent cell.

Therefore, the correct answer is option b) 1, as it represents the default step value in MS Excel's Fill Series dialog box.