Which of the following is a felony offense in Colorado?

Soliciting the illegal taking of big game is classified as a felony offense in Colorado due to the significant legal implications it carries regarding wildlife conservation and management. This act involves not only an attempt to engage in illegal hunting, but also the intent to facilitate or promote such illegal activities, which can have detrimental effects on wildlife populations and ecosystems.

In contrast, while the other options may also have legal consequences, they typically involve lesser offenses. Using a firearm while hunting, for instance, might lead to misdemeanor charges or penalties if done improperly, but it does not inherently indicate intent to promote illegal activities. Killing small game without proper licensing or hunting on private land without permission can result in fines or misdemeanor charges as well. However, these actions do not have the same level of severity in terms of intent and the impact on wildlife as soliciting the illegal taking of big game does, which is why the latter is treated more harshly under Colorado law.