Question

They had to climb ___ the steep pathway to reach the top

Answer 1

They had to climb _up_ the steep pathway to reach the top
UP is used either as a preposition or as an adverb particle. It acts as an adverb particle when we use phrase 'stand up' and 'blow up'.  In above statement, it acts as an adverb particle.

Answer 2

Hi friend,

The answer is UP.

They had to climb UP the steep pathway to reach the top.
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Hope it helped you out!!!!!!