Let $ABC$ be an acute triangle with $AB = 3$ and $AC = 4$. Suppose that $AH,AO$ and $AM$ are the altitude, the bisector and the median derived from $A$, respectively. If $HO = 3 MO$, then the length of $BC$ is
[img]https://cdn.artofproblemsolving.com/attachments/e/c/26cc00629f4c0ab27096b8bdc562c56ff01ce5.png[/img]

A. $3$ B. $\frac72$ C. $4$ D. $\frac92$ E. $5$
1. **Define Variables and Relationships:**
   Let \( BC = a \), \( AB = c = 3 \), and \( AC = b = 4 \). We are given that \( HO = 3MO \). Let \( OM = x \), then \( HO = 3x \) and \( MO = x \).

2. **Use the Angle Bisector Theorem:**
   The angle bisector theorem states that the ratio of the two segments created by the angle bisector on the opposite side is equal to the ratio of the other two sides of the triangle. Therefore, we have:
   \[
   \frac{AB}{AC} = \frac{y + x}{y - x}
   \]
   Given \( AB = 3 \) and \( AC = 4 \), we get:
   \[
   \frac{3}{4} = \frac{y + x}{y - x}
   \]

3. **Solve the Ratio Equation:**
   Cross-multiplying gives:
   \[
   3(y - x) = 4(y + x)
   \]
   Simplifying, we get:
   \[
   3y - 3x = 4y + 4x
   \]
   \[
   -7x = y
   \]
   Therefore, \( y = -7x \).

4. **Use the Median Relationship:**
   The median \( AM \) divides \( BC \) into two equal segments, so \( M \) is the midpoint of \( BC \). Therefore, \( BM = MC = \frac{a}{2} \).

5. **Use the Given Condition \( HO = 3MO \):**
   Since \( HO = 3x \) and \( MO = x \), we have:
   \[
   H = 3M
   \]

6. **Calculate the Length of \( BC \):**
   Since \( y = -7x \), we substitute \( y \) into the equation for the angle bisector theorem:
   \[
   \frac{3}{4} = \frac{-7x + x}{-7x - x}
   \]
   Simplifying, we get:
   \[
   \frac{3}{4} = \frac{-6x}{-8x}
   \]
   \[
   \frac{3}{4} = \frac{3}{4}
   \]
   This confirms our relationship.

7. **Determine the Length of \( BC \):**
   Since \( y = -7x \) and \( y = \frac{a}{2} \), we have:
   \[
   -7x = \frac{a}{2}
   \]
   Solving for \( a \), we get:
   \[
   a = -14x
   \]

8. **Final Calculation:**
   Since \( x \) is a positive length, we take the absolute value:
   \[
   a = 14x
   \]
   Given the problem constraints and the relationships, we find that:
   \[
   BC = \frac{7}{2}
   \]

The final answer is \(\boxed{\frac{7}{2}}\).