Select the option which is NOT an antonym of another word by way of adding the prefix ‘in-‘. 

The correct answer is "insidious"

Key Points

• The prefix "in-" is often added to words to give them the opposite meaning. For example, "unhappy" means not happy, "impossible" means not possible, "illogical" means not logical, and so on.
• In the given options, "inoffensive" means not offensive, "innumerable" means too many to be counted, and "inscrutable" means not easily understood. These words are all antonyms of their base words with the addition of the prefix "in-".
• However, "insidious" means proceeding in a gradual, subtle way, but with harmful effects. Its opposite would be "straightforward" or "open". Adding the prefix "in-" to "sidious" does not result in a valid word, making "insidious" the option that is not an antonym of another word by way of adding the prefix "in-".
• Therefore, the correct option is "insidious", which is not an antonym of another word by way of adding the prefix "in-".

Correct answer: insidious