With commercial classes, the function source code is usually________

The correct answer is: C. provided on a disk.

Commercial classes are typically sold as software packages, which are typically distributed on disks or other storage media. The function source code is usually provided in the form of text files, which can be compiled and executed by the user.

Option A is incorrect because function source code is typically not printed on high-quality paper. Option B is incorrect because function source code is typically well-written and well-documented. Option D is incorrect because function source code is typically not provided in object form.