At first, I thought this question was going to be very easy.  After reading the list of choices about indirect characterization in "The Sniper," I had a hard time choosing between speech and reactions from other characters.  

I believe that speech is used the least.  Throughout the entire story, the sniper says exactly two words.  "I'm hit."  The text says that he muttered the words too, which actually tells you a lot about the sniper's high tolerance for pain.  If I got shot, I would be saying a lot more than two words.  And I for sure wouldn't be muttering them.  

The other characters in the story do not react too much to the sniper, which is why I thought it was a good candidate for the correct answer.  The other characters don't react, because they can't.  They're dead. Except for the enemy sniper.  The enemy and the protagonist play a bit of cat and mouse with each other.  I have always gotten the feeling that the enemy sniper knows that his quarry is dangerous, because he waits for a head shot.  He knows that an arm shot is not enough to put the main character sniper out of commission.  That piece of information is important to knowing the sniper. The reader also learns that the main protagonist takes some pleasure from outwitting the actions of his enemy.  It's why he smiles right before his kill shot.  

He [the enemy] was now standing before a row of chimney pots, looking across, with his head clearly silhouetted against the western sky. The Republican sniper smiled and lifted his revolver above the edge of the parapet.