The phrase "trudge along" suggests a slow, laborious, and weary walking, typically due to exhaustion or fatigue. In this sentence, the travellers have completed half of the journey, and the word "trudge" indicates a tired and heavy way of walking. Option D, "walk wearily," closely is closest in meaning to "trudge along." "Walk wearily" means walking with a sense of tiredness or weariness and is close to the idea that the travellers could only proceed with a certain level of fatigue after completing half of the journey.