A Digital Developer has a site export file on their computer that needs to be...

To import a site export file into a sandbox, the recommended method involves using the Site Import & Export Business Manager module:

Option B: Upload and import the file using the local option within the Site Import & Export Business Manager module is the correct answer. This option allows the developer to upload a site export file directly through the Business Manager interface and then import it into the sandbox. This is the most straightforward and typical method for updating sandbox environments with new data or configurations.

Other options like connecting remotely (A), using the Impex WebDAV directory (C), or using the Static WebDAV directory (D) are either incorrect methods or involve incorrect directories for site import activities.