Ad hoc arbitration is a type of arbitration that is not administered by a formal arbitral institution (like the London Court of International Arbitration or the Singapore International Arbitration Centre). Instead, the parties themselves are responsible for managing all aspects of the proceeding. This includes:
Selecting the arbitrators.
Determining the applicable procedural rules (they can create their own, adopt existing ones like the UNCITRAL rules, or let the tribunal decide).
Deciding the language and seat of arbitration.
Managing all administrative aspects.
All the statements (a), (b), and (c) describe different facets of this party-driven arrangement. Therefore, the most comprehensive answer is (d).