To identify the error in the given sentence, let's go through each part:
Option A) a - This option is correct because "the travel" is a singular noun, so it should be preceded by the indefinite article "a" instead of the definite article "the."
Option B) b - This option is incorrect because "across the sandy desert" is a prepositional phrase that provides additional information about the travel. There is no error in this part.
Option C) c - This option is incorrect because "was an exhausting one" is a complete and grammatically correct sentence. There is no error in this part.
Option D) d - This option is incorrect because "for the caravan" is a prepositional phrase that provides additional information about who the travel was exhausting for. There is no error in this part.
Option E) e - This option is incorrect because there is an error in part a.
Therefore, the correct answer is A) a.