Select the most appropriate word to fill in the blank.
The Internet is an _________ resource for teachers.

The correct option to fill in the blank is (a) invaluable.
Explanation: "Invaluable" means extremely useful, which fits the context of describing the Internet as a resource.
Meanings of other options:
· Invaluable → Extremely valuable (Correct).
· Antique → Something old or vintage.
· Auspicious → Indicating success or good fortune.
· Inimical → Harmful or hostile.