Answer and explanation:
The correct answers are:
"Some wounded thing, by the evidence a large animal, had to thrashed about in the underbrush."
He was able to tell an animal had been there based on how the underbrush looked. This shows that he has some experience tracking.
"The jungle weeds were crushed down and the moss was lacerated; one patch of weeds with stained crimson."
He noticed small details that likely indicated this is where the animal had bled significantly. Again this shows that he has experience tracking.
"A small, glittering object not far away caught Rainford's eye and he picked it up. It was an empty cartridge."
Rainsford knew that this spot is likely where the animal was shot. He again is using his keen attention to detail that shows that he has had experience tracking. Because of this, he was able to find the cartridge of the bullet that caused the animal to bleed.