Step 1: Compare the substituents on N
Two CH$_3$ groups, one CH$_3$, none, and one phenyl.
Step 2: Rank
More electron donation means a stronger base, and phenyl withdraws electron density by resonance. This gives the order in option (B).
Final Answer:
Option (B) lists the correct decreasing basic strength.
\[ \boxed{(\text{CH}_3)_2\text{NH} > \text{CH}_3\text{NH}_2 > \text{NH}_3 > \text{C}_6\text{H}_5\text{NH}_2} \]