1. Identify the compounds

We need the decreasing order of basic strength among:

• $NH_3$ (ammonia)
• $C_2H_5NH_2$ (ethylamine, a primary amine)
• $(C_2H_5)_2NH$ (diethylamine, a secondary amine)

2. Recall the factor affecting basic strength

A base is stronger if the lone pair on nitrogen is more easily available for donation.

Alkyl groups like $C_2H_5-$ show a $+I$ effect (electron-donating inductive effect), which increases electron density on nitrogen and makes the amine more basic than ammonia.

So, compared to $NH_3$:

• $C_2H_5NH_2$ is more basic than $NH_3$
• $(C_2H_5)_2NH$ is generally more basic than $C_2H_5NH_2$ in aqueous medium among these choices

3. Compare the three compounds

• $NH_3$ has no alkyl group, so least electron donation.
• $C_2H_5NH_2$ has one ethyl group, so nitrogen gets more electron density.
• $(C_2H_5)_2NH$ has two ethyl groups, so even greater $+I$ effect, making the lone pair most available.

Hence,

4. Match with the options

Option C states:

which matches our result.

5. Final answer

The decreasing order of basic strength is:

So, the correct option is C.