File Transfer Protocol (FTP)

Correct Answer: Username and password

The best method to prevent unauthorized access to an FTP server is with a 'Username and Password'. This is a common method that provides a basic level of security by requiring each user to provide a set of authentication details that should be known only to them. 'User's IP Address only' is ineffective because IP addresses can be easily spoofed or could change if the user has a dynamic IP address. 'Username only' is not sufficient because without a password, anyone who knows or guesses a username could gain access. 'Anonymous login' is the least secure option because it does not require any credentials, thus allowing anyone to access the FTP server.

Correct Answer: FTPS

The best protocol for an IT administrator to use when transferring sensitive files over FTP for data security is FTPS. FTPS is like FTP, but it adds a layer of security. FTPS encrypts the data being transmitted which provides a secure connection for transferring sensitive files, preventing unauthorized access. On the other hand, FTP is not secure, it transfers data in clear text, which can be easily intercepted and read. Similarly, TFTP (Trivial File Transfer Protocol) is a simpler version of FTP that also does not provide any security. And lastly, HTTP is also not suitable for this purpose as it is mainly used for transmitting hypertext over the internet and does not provide the required level of security for transferring sensitive files, unless it's HTTPS which is more secure.

Correct Answer: Insufficient server storage

The most plausible cause for a file upload to an FTP server failing continuously, especially during non-peak times, would be 'Insufficient server storage'. During non-peak times, network traffic is generally low and hence network latency is unlikely. 'Incorrect file type' could also cause upload failures but it is less likely to cause continuous failures unless the same wrong file type is being attempted repeatedly. Similarly, if the server was unavailable, no operations (not just uploading files) would be possible. Hence, among the provided options, the insufficient storage on the server seems to be the most probable reason, as the FTP server might not have enough storage space to accommodate the new file being uploaded.