"i have my doubts," i replied; "and i was silly enough to pay the full amontillado price without consulting you in the matter. you were not to be found, and i was fearful of losing a bargain."
the narrator was worried he would lose a bargain.
the narrator wants his fortunato to feel superior.
the narrator was upset by paying too much for the amontillado.
the narrator could not find fortunato when he needed him.

You have not specifically stated the problem in your question.

Is there one correct sentence (matching the small piece adapted from a novel or so) and you are asked to find it ?

Doesn't seem so. Because I think there are three correct sentences and one wrong sentence giving a fact unrelated to the passage.
So, that makes the question "Find out the wrong sentence out of the given four.".

Let's proof-read each and every sentence by matching the facts given in the given passage. In this way, we will eventually bump at the wrong sentence and our proof-reading will help us know it.

PROOF-READING:

a. The narrator was worried he would lose a bargain.

It's correct. Because the narrator states in the end of his speech that he was scared of losing a bargain. "and I was fearful of losing a bargain" - quoted. 

Urgh!! Not anywhere near the wrong sentence. Don't lose hope. Who knows that the wrong sentence is just hiding beneath our nose?  Maybe the coming sentence will be our incorrect sentence.

b. The narrator wants his Fortunato to feel superior.

It's incorrect. For there is no mention of the object "Fortunato" in the passage. Plus, the writer never tells anything about he wanting anyone to feel superior. Maybe, as a hidden fact, it might be considered true.

Yes Our long wanted desire fulfilled. We found out the sentence we were struggling so long for!! Still, it is better to check the other sentences - whether they are correct, incorrect or what.

c. The narrator was upset by paying too much for the Amontillado.

It's absolutely correct. In fact, the narrator's whole speech is about it. When he could not suddenly find someone who is stated in the passage as "you" - second person, he was not left with any options but to pay too much for the Amontillado otherwise, he was fearful that he could lose a bargain. "I was silly enough to pay the full Amontillado price without consulting you in the matter." - quoted.

d. The narrator could not find Fortunato when he needed him.

It's too correct. It is stated by the narratter that he could not find "you" who is actually Fortunato when he needed him. It is quite imminent that the narrator was quite highly dependent on him, or he ended up making silly decisions which he regrets after he completes doing it.

So, that leaves on the sentence, serially B being the incorrect one.