Step 1: Idea
Work with the ratio of the given mass to one mole of the gas. If $17$ g of $\text{NH}_3$ holds $6.022\times10^{23}$ molecules, then a smaller mass holds a proportionally smaller number.
Step 2: Proportion method
Molar mass of $\text{NH}_3$ = $14 + 3 = 17$ g. So the fraction of a mole is $\dfrac{2.125}{17}$. Since $17\times 0.125 = 2.125$, the fraction is exactly $\dfrac{1}{8}$.
Step 3: Count the molecules
One eighth of Avogadro number is
\[ \frac{6.022\times10^{23}}{8}=0.75275\times10^{23}=7.527\times10^{22} \]
Step 4: Compare with options
$6.022\times10^{23}$ would need 17 g and $3.011\times10^{23}$ would need 8.5 g. Both are much larger than 2.125 g. The value $1.505\times10^{22}$ is only 0.025 mol, which is 0.425 g. The match is option (D).
Final Answer:
The sample is 0.125 mol of $\text{NH}_3$, which holds $7.527\times10^{22}$ molecules, option (D).
\[ \boxed{7.527\times10^{22}} \]