The correct answer is: A. Alt + V
When the default invoice mode is on, you can enter a sales or purchase as a voucher by pressing Alt + V. This will open the voucher entry screen, where you can enter the details of the transaction.
Option B, Ctrl + V, is used to paste text. Option C, Alt + F8, is used to open the Find and Replace dialog box. Option D, Alt + F9, is used to update the current field.
I hope this helps! Let me know if you have any other questions.