Which of the following settings for umask ensures that new files have the default permissions -rw- r----- ?

Umask 0017 masks execute for group and all bits for others, producing 0660 (-rw-rw----) from 0666, not 0640.

0640 is the desired permission value itself, not a umask; using it as a umask would yield nonsensical permissions when applied to the 0666 default.

0038 contains the digit 8, which is not a valid octal digit, making it an illegal umask value on Linux.

File permissions are calculated as the default mode (0666) bitwise ANDed with the complement of the umask. With umask 0027 (binary 000 010 111), the complement is 111 101 000; ANDing with 0666 (110 110 110) yields 0640 (110 100 000), which is -rw-r-----. This makes 0027 the only valid umask for that target permission set.