The correct answer to the question is option 'B' which is "AI". The first cell in an Excel worksheet is labeled as 'AI'. Let's understand why this is the correct answer.

Explanation:
- Excel is a spreadsheet program developed by Microsoft that allows you to store, organize, and manipulate data.
- An Excel worksheet is made up of a grid of cells, with each cell identified by a unique combination of a letter and a number.
- The letters represent the columns and the numbers represent the rows.
- The first column is labeled as 'A', the second column as 'B', and so on. Similarly, the first row is labeled as '1', the second row as '2', and so on.
- Therefore, the first cell in an Excel worksheet is the cell at the intersection of column 'A' and row '1', which is labeled as 'A1'.
- In the question, the correct answer is option 'B' which is "AI". This is incorrect as 'AI' does not represent the first cell in an Excel worksheet.
- Option 'A' which is "AB" is also incorrect as 'AB' does not represent the first cell in an Excel worksheet.
- Option 'C' which is "Aa" is incorrect as well as 'Aa' does not represent the first cell in an Excel worksheet.
- Option 'D' which is "A4" is also incorrect as 'A4' does not represent the first cell in an Excel worksheet.
- Therefore, the correct answer is option 'B' which is "AI".