The strongest base in gaseous phase among the following is:
In the gaseous phase, tertiary amines like (CH₃CH₂)₃N have the highest basicity due to maximum electron donation from three alkyl groups, as per the PDF.
Ref: NCERT Class 12 Chemistry > Chapter 13: Amines > Topic: Reaction with Nitrous Acid and Diazonium Salts - Part 1