The correct answer is A. slide.
A slide is a gradual downward movement, especially on a slippery surface. It is the opposite of a climb.
A decline is a gradual decrease in size, amount, or value. It is the opposite of an increase.
A fall is a sudden and uncontrolled movement downwards. It is the opposite of a rise.
A descent is a journey downwards, especially from a higher to a lower level. It is the opposite of an ascent.
In the context of the question, the climbers found the ascent nerve breaking. This means that they found the climb to be very difficult and challenging. A slide is a gradual downward movement, especially on a slippery surface. This is consistent with the climbers’ description of the ascent as being difficult and challenging. The other options are not consistent with the climbers’ description of the ascent.