Step 1: Concept
In a homologous series, any two successive members differ by a $-CH_2-$ group.
Step 2: Analysis
- 3rd alkane: Propane ($C_3H_8$)
- 4th alkane: Butane ($C_4H_{10}$)
Step 3: Calculation
Difference = Molar mass of $CH_2 = 12 (C) + 2 \times 1 (H) = 14$ g/mol.
Step 4: Conclusion
The difference is 14 g/mol.
Final Answer:(B)