The correct activity in Automation Studio to decrypt a file placed on the SFTP for import is the File Transfer Activity. This activity is used to securely transfer and manage files, including options for unzipping and decrypting files. It prepares files for further actions within Automation Studio, such as data import.
The correct answer is Data Extension, Audience Studio, API Event, and Date Based Event. Entry source types for Journey Builder encompass these sources, which include various methods for initiating journeys based on data and events. There is no mention of email lists or CloudPages in the standard entry sources for Journey Builder.