What is the correct syntax to turn a VPN debug on and create new empty debug files'?

The correct command to simultaneously enable VPN debugging and truncate (empty) the existing debug log files is vpndebug truncon. The truncon parameter is a compound argument that combines trunc (truncate) and on. Option B, despite containing a common typo (vpn debug instead of the correct vpndebug), is the only choice that uses the correct and specific truncon parameter to perform both actions as requested by the question. This makes it the most plausible intended answer.

A. vpndebug trunc on This syntax is invalid. The vpndebug command does not accept trunc and on as sequential arguments in this manner. The combined action requires the specific truncon keyword.
C. vpn debug trunkon This is incorrect due to the misspelling of the parameter as trunkon. The correct parameter is truncon. "Trunk" has no relevance to this command's function.
D. vpn kdebug on This command is incorrect. The tool for VPN daemon debugging is vpndebug. Kernel-level debugging is typically performed with fw kdebug, which is a different utility for a different purpose.

1. Check Point Support Center, sk33327: "How to debug VPN - IKE and IPsec - on Check Point Security Gateway". This official knowledge base article explicitly lists and describes the vpndebug command options. In the "Solution" section, it states:

vpndebug on - Turns on the debug.

vpndebug trunc - Truncates the debug files...

vpndebug truncon - Truncates the debug files and turns on the debug.

This source directly validates that truncon is the correct parameter for the combined action.

2. Check Point R81.10 Security Gateway and Management CLI Reference Guide. In the section detailing command-line utilities, the guide describes the vpndebug command.

Page 1081: The guide lists the usage and parameters for vpndebug, confirming that truncon is a valid parameter that "Truncates debug log files and starts debug." This confirms the functionality described in the question maps directly to the truncon parameter.