I opened the bag and packed the boots in; and then, just as I was going to close it, a horrible idea occurred to me. Had I packed my toothbrush? I don't know how it is, but I never do know whether I've packed my toothbrush.
My toothbrush is a thing that haunts me when I'm travelling, and makes my life a misery. I dream that I haven't packed it, and wake up in a cold perspiration, and get out of bed and hunt for it. And, in the morning, I pack it before I have used it, and it is always -the last, thing I turn out of the bag; and then I repack and forget it, and have to rush upstairs for it at the last moment and carry it to the railway station, wrapped up in my pocket-handkerchief.
What makes his life miserable whenever he undertakes travelling?
The toothbrush
The passage explicitly states, "My toothbrush is a thing that haunts me when I'm travelling, and makes my life a misery." This sentence directly identifies the recurrent issue of the toothbrush as the primary cause of the narrator's distress during travel.
(A) Going to railway station: This is described as a logistical consequence of forgetting the toothbrush and having to retrieve it at the last moment, rather than the fundamental source of misery itself.
(C) Packing his bag: While packing is the context in which the anxiety arises, the act of packing itself is not the cause of misery. The distress stems specifically from the uncertainty and repeated issues concerning the toothbrush during the packing process.
(D) Bad dreams: The passage indicates that bad dreams are a symptom or manifestation of the narrator's anxiety about forgetting the toothbrush ("I dream that I haven't packed it, and wake up in a cold perspiration"). They are a result of the underlying worry, not the root cause of the misery.