Choose the correct word to complete the sentence:
Navigating through the dense forest, the hikers encountered ______________ terrain, making the trek more challenging and adventurous.

The correct answer is (d) treacherous.
Explanation:
"Treacherous" means dangerous or difficult, which fits well with the description of a challenging and adventurous trek. The other options do not convey the sense of danger needed for this context.
Meanings of all options:
· Predictable: Something that can be easily foreseen or expected.
· Monotonous: Dull or repetitive.
· Tranquil: Calm or peaceful.
· Treacherous: Dangerous or hazardous.
Correct sentence:
"Navigating through the dense forest, the hikers encountered treacherous terrain, making the trek more challenging and adventurous."