Correct Answer: Complex

Solution : The second option is the correct choice.

The most appropriate synonym for elaborate is complex. Elaborate refers to detailed and intricate explanations or plans. Similarly, complex denotes a high level of intricacy or difficulty, making it a suitable synonym in this context.

The meanings of the other options are as follows:

• Easy: It indicates simplicity or a lack of difficulty.
• Simple: It suggests a lack of complexity or intricacy.
• Plain: It refers to simplicity without embellishments or complexity.

Therefore, the correct answer is complex.