To limit a PROC MEANS analysis to the variables Boarded, Transfer, and Deplane, we need to specify the variables in the procedure statement.

Option A "by boarded transfer deplane" is used to specify the variables used in the BY statement for the procedure. This is not used to limit the variables in the analysis.

Option B "class boarded transfer deplane" is used to classify variables. This option is not used to limit the variables in the analysis.

Option C "output boarded transfer deplane" is used to specify the variables to be included in the output dataset. This option is not used to limit the variables in the analysis.

Option D "var boarded transfer deplane" is used to specify the variables to be analyzed in the PROC MEANS statement. This option is correct as it limits the analysis to the specified variables.

Therefore, the answer is: D. var boarded transfer deplane;