Let $ a,b,c,d$ be rational numbers with $ a>0$. If for every integer $ n\ge 0$, the number $ an^{3} \plus{}bn^{2} \plus{}cn\plus{}d$ is also integer, then the minimal value of $ a$ will be

$\textbf{(A)}\ 1 \qquad\textbf{(B)}\  \frac{1}{2} \qquad\textbf{(C)}\  \frac{1}{6} \qquad\textbf{(D)}\ \text{Cannot be found} \qquad\textbf{(E)}\ \text{None}$
1. **Step 1: Analyze the given polynomial for specific values of \( n \)**
   - Let \( n = 0 \):
     \[
     P(0) = d
     \]
     Since \( P(0) \) is an integer, \( d \) must be an integer.

   - Let \( n = 1 \):
     \[
     P(1) = a(1)^3 + b(1)^2 + c(1) + d = a + b + c + d
     \]
     Since \( d \) is an integer and \( P(1) \) is an integer, \( a + b + c \) must also be an integer.

   - Let \( n = 2 \):
     \[
     P(2) = a(2)^3 + b(2)^2 + c(2) + d = 8a + 4b + 2c + d
     \]
     Since \( d \) is an integer and \( P(2) \) is an integer, \( 8a + 4b + 2c \) must also be an integer.

2. **Step 2: Derive conditions from the polynomial for \( n = 2 \)**
   - Consider the difference:
     \[
     (8a + 4b + 2c + d) - 2(a + b + c + d) = 8a + 4b + 2c - 2a - 2b - 2c = 6a + 2b
     \]
     Since \( 8a + 4b + 2c + d \) and \( a + b + c + d \) are integers, \( 6a + 2b \) must be an integer.

3. **Step 3: Derive conditions from the polynomial for \( n = 3 \)**
   - Let \( n = 3 \):
     \[
     P(3) = a(3)^3 + b(3)^2 + c(3) + d = 27a + 9b + 3c + d
     \]
     Since \( d \) is an integer and \( P(3) \) is an integer, \( 27a + 9b + 3c \) must also be an integer.

   - Consider the difference:
     \[
     (27a + 9b + 3c + d) - 3(a + b + c + d) = 27a + 9b + 3c - 3a - 3b - 3c = 24a + 6b
     \]
     Since \( 27a + 9b + 3c + d \) and \( a + b + c + d \) are integers, \( 24a + 6b \) must be an integer.

4. **Step 4: Combine the conditions**
   - From \( 6a + 2b \) being an integer and \( 24a + 6b \) being an integer, we can derive:
     \[
     4(6a + 2b) - (24a + 6b) = 24a + 8b - 24a - 6b = 2b
     \]
     Since \( 2b \) is an integer, \( b \) must be an integer.

   - From \( 6a + 2b \) being an integer and \( b \) being an integer:
     \[
     6a + 2b - 2b = 6a
     \]
     Since \( 6a \) is an integer, \( a \) must be a rational number such that \( 6a \) is an integer. Therefore, \( a \) must be of the form \( \frac{k}{6} \) where \( k \) is an integer.

5. **Step 5: Determine the minimal value of \( a \)**
   - Since \( a > 0 \), the smallest positive value for \( a \) is when \( k = 1 \):
     \[
     a = \frac{1}{6}
     \]

Conclusion:
The minimal value of \( a \) is \( \boxed{\frac{1}{6}} \).