Final answer:
Thomas should navigate to Gear > Account and settings > Sales > Online Delivery to customize how invoices appear in emails and attach a PDF.
Step-by-step explanation:
Thomas can change how invoicing details are displayed and delivered to his customers by navigating to the correct settings in his accounting or invoicing software. The correct path for Thomas to make this change would be b) Gear > Account and settings > Sales > Online Delivery > How your invoice appears in emails. This path allows Thomas to customize how the invoice is presented in the email body and to also attach a PDF copy. After navigating to this setting, he'll need to choose the option that both shows the invoice details in the email and attaches a PDF of the invoice.
To make the desired change, Thomas would need to navigate to option d) Gear > Account and settings > Sales > Sales form content > How your invoice appears in emails. Here, he can customize the content of his invoice emails, including attaching a PDF of the full invoice details. By choosing this option, his customers will see the invoice details directly in the email itself and also have a PDF attachment for their convenience.