1. The size of materials being placed in the composting system.
2. The carbon to nitrogen (C:N) ratio of materials placed in the composting system.
3. The level of management/attention paid to the composting process.
4. The intended use for finished compost.
5. All of the above.

You are correct!  The answer is "e.  All of the above".

A backyard composting system may yield finished soil-like compost in two to three months. This can be accomplished by using small organic pieces less than 2", a mixture of organic materials with a carbon to nitrogen ratio of approximately 30:1, and an actively managed system (where the pile is turned one to two times per week and monitored for correct moisture content and temperature). More time is needed to produce compost if less attention is paid to material size, C:N ratio, and active management.

Compost that will be used as a mulch will be ready for use more quickly than compost that will be incorporated into the soil for immediate planting.