why he is also described as "dog-like" at this point.

Jack is very much described as animal-like at this point in the book; for example, the author also tells us that Jack had "flared nostrils," which is also an image that one would most commonly associate with an animal such as a dog. When Jack is startled by the cry of a bird, the author continues with this animal-themed imagery, describing Jack as " ape-like."

You could interpret this use of imagery in many ways. I would suggest perhaps looking into the further development of Jack's character within the book. You could argue, for example, that perhaps Golding chose this kind of imagery because he wanted to show the reader already in chapter 3 the kind of animal-like person Jack is and how he often lacks human qualities such as empathy.

Chapter nine of Lord of the Flies, by William Golding, is the story of boys who are civilized in the beginning of the novel but become savages by the end. What happens in chapter nine is a huge step toward savagery. The passage in question is below:

The sticks fell and the mouth of the new circle crunched and screamed. The beast was on its knees in the center, its arms folded over its face. It was crying out against the abominable noise something about a body on the hill. The beast struggled forward, broke the ring and fell over the steep edge of the rock to the sand by the water. At once the crowd surged after it, poured down the rock, leapt on to the beast, screamed, struck, bit, tore. There were no words, and no movements but the tearing of  teeth and claws.

Your question is what image the circle of boys who kill Simon (the beast in the center of the circle) is most like. The imagery of this passage does not depict a crowd *though it is mentioned) or a horseshoe; the boys are in a ring that collapses in on Simon, but a ring is not a particular image here. That leaves mouth and beast. Visually, a mouth is an effective image for a collapsing circle, however, this circle of boys has a mouth that screams and crunches, it “leapt onto the beast [Simon]” (which a mouth cannot do), bites with teeth and tears with claws.

The imagery suggests that this circle of boys is a literal beast (choice D)—which is, ironically, exactly what Simon wanted to tell them. He tried to tell them that they were the beast, but it seems they have already discovered that. 

e. Auditory

Among the choices, the two dominant types of imagery are visual and auditory; however, there are more auditory images than visual near the end of Chapter Eleven of Lord of the Flies, and they are certainly shattering and unnerving images, as well.

With the death of Simon and the stealing of Piggy's glasses, all vestiges of civilization have been destroyed by Jack's band of savages. Hoping to appeal to the remnants of civility in Jack, Piggy grabs the conch with the determination to appeal to Jack to restore some order on the island. Surprisingly, when Piggy and others approach, the savages grow silent. However, they soon begin booing; then, when he calls them "a pack of painted Indians" in contrast to the "sensible" Ralph, the cacophony of savagery recommences:

The booingrose...
Again the clamor..."Zup!"
Ralph shouted against the noise.
... bristled with spears.
The storm of sound beat at them, an incantation of hatred.
The rock bounded
...the sea breathed...in a long, slow sigh, the water boiled...
This time the silence was complete...
...Jack [was]...screaming
... his spear... tore the skin
... the tribe screaming now
A great noise as of sea gulls rose...
...[Ralph] was crashing through foliage....
...the tribe returned noisily...
The yelling ceased, and Samneric lay looking up in quiet terror.