Correct Answer: gripping

Solution : The correct choice is the second option.

Explanation:
Intriguing means something that is fascinating, interesting, or captivating, often prompting curiosity or interest. Gripping is the synonym for intriguing, signifying something that's engaging or compelling, holding one's attention.

The meanings of the other options are as follows:

• Monotonous refers to something that is repetitive, dull, lacking in variation, or exciting.
• Simultaneous denotes things occurring at the same time.
• Investigating involves examining, researching, or looking into something to find out more about it.

Therefore, the correct answer is gripping.