What characteristic of Harry's makes us think that he might be a trustworthy narrator?

He tells us when he has minor flaws such as being afraid.

He is the one telling the story the whole time.

He is younger than the Professor, so he must be more trustworthy.

He focuses on himself, so we can believe him.

He tells us when he has minor flaws such as being afraid.

Explanation:

One of the most common issues making a narrator untrustworthy is his/her bias toward oneself and toward other characters of the story whom he/she likes or does not like.

Most of the time bias is in favor of oneself, in rare cases it may be against oneself - blaming oneself excessively.

Telling one's own minor and/or major flaws is only one of many characteristics to make a narrator trustworthy.

All other options are either insignificant for adjudging him as a trustworthy narrator, or opposite of what makes him trustworthy and neutral.

Second and third options are insignificant (do not contribute in making him neutral narrator)

Fourth option is incorrect because focusing on oneself makes a narrator biased and hence untrustworthy.