The main challenge of international HRM is:

International Human Resource Management (IHRM) involves managing HR activities across national borders, which introduces significant complexities beyond domestic HRM. These complexities stem from diverse environments and expectations.

Option B: Managing cultural differences and local labor laws is the main challenge. Cultural differences impact communication, motivation, leadership styles, and team dynamics. Simultaneously, navigating varied and often complex local labor laws, regulations, and employment practices across different countries is critical for compliance and effective operations.

• Option A: Recruiting locally is a task within IHRM, but the challenge lies in doing so effectively amidst cultural and legal nuances.
• Option C: Using English only is often insufficient for effective international communication and can lead to misunderstandings, but it's not the overarching main challenge.
• Option D: Reducing travel costs is a logistical and financial concern, secondary to the core HR complexities of managing people across diverse cultural and legal landscapes.