One effect of decreasing wolf populations in north america is:

The decreasing wolf populations in North America can result to the range expansion of coyotes

Wolves and coyotes usually compete for prey, especially the smaller mammals. Wolves do not actively hunt coyotes but do attack them when the latter ventures into their territory.

With decreasing wolf populations, coyotes can then venture and hunt more freely. However, the coyotes cannot take the place of the wolves in helping maintain balance in the ecosystem in which they belong to because of their preference for smaller mammals.

Wolves can help regulate an ecosystem's larger mammals such as the ungulate populations, as they do not prey on the smaller mammals as much as coyotes do. This leaves more prey available for mid-level carnivores like foxes. Moreover, wolves also leave behind carcasses of their prey, which become food for other animals such as foxes, badgers, raptors, coyotes and other scavengers.

Ithink the answer is d.