The logical starting point here is C — "When for days you have been going through a mountain pass..." — because it sets the scene and introduces the expectation of reaching a plain. This opening builds anticipation for what is to come after the long journey.
Next, D — "Surely after this you will see the plain." — follows naturally from C, as it expresses the certainty or assumption the traveler holds after the long trek. It is a direct continuation of the thought process introduced in C.
However, the text takes an unexpected turn with B — "No, the path winds on and another mountain bars your way." This contrasts with the hope expressed in D, revealing disappointment and an obstacle instead of the expected plain.
Finally, A — "But instead you are faced with another huge crag and the weary trail continues." — strengthens the imagery introduced in B and adds a vivid description of the ongoing challenge. This conclusion fits perfectly, reinforcing the endurance needed for the journey.
Thus, the correct sequence is C → D → B → A, which corresponds to option (a) CDBA.