Find a common denominator (LCD of 10 and 4 is 20), then subtract.<br><br>
&bull; \( \frac{9}{10} = \frac{18}{20} \)<br>
&bull; \( \frac{3}{4} = \frac{15}{20} \)<br>
&bull; \( \frac{18}{20} - \frac{15}{20} = \frac{3}{20} \)<br>
&bull; \( \frac{3}{20} \) is already in simplest form.<br><br>
Final Answer: C: \( \frac{3}{20} \)