Final answer:
Mayfair Mobile is adopting an organic structure by decentralizing decision-making to lower-level managers, which is suitable for quickly responding to rapid environmental changes.
Step-by-step explanation:
When Mayfair Mobile's external environment is changing rapidly and it responds by decentralizing decisions to lower-level managers to react faster, it is adopting an organic structure. This type of structure is characterized by flexibility, decentralized authority, and adaptive practices that respond quickly to environmental changes and empower lower-level managers to make decisions. In contrast, a mechanistic structure is more hierarchical and rigid, a bureaucratic structure is characterized by clear divisions of labor and explicit rules, and oligarchic and formal structures are generally associated with a concentration of power and a high degree of formalization.