I agree that D is the best answer because intranets and extranets are based on the needs of the company.  Intranet certainly is company-specific, because it consists of internal information customized to the needs of employees and people within the company.  Extranet is also company-specific because it is customized to a select group of the company's customers; the company uses an extranet to do business with a select group of its customers.  Each customer has its own extranet site that only that customer and the company can access.  The extranet is not meant to be accessible to or used by the general public.

Systems programmer, at least for the purposes of the CPA exam, is usually what we call a systems administrator in real life.  Systems administrators (called "sysadmins" by IT folks) deal with maintaining operating systems, hardware and networks, and they do day-to-day tasks of keeping the system running.  In real life, a systems programmer is someone who creates and modifies code on the level of the operating system or compiler (below the application/software level).  Systems programmers and sysadmins sometimes do write small programs that are customized to the company, but most of what they do are specific to the type of operating systems and hardware used by the company.  So I would say A is the second best answer, but D is still better.

Application programmer develops any kind of software (e.g., word processing programs, spreadsheet programs, data processing programs, etc.).  This software is not necessarily customized for the company.  The application developer could be writing the software for sale to the public.

Same deal with internet developer, which really is a subset of application programmer.  An internet developer writes web-based applications (while application programmer also writes non-web-based applications) that are not necessarily customized for the company.  Web applications can be and usually are written for sale to the public.