user
Return your final response within \boxed{}. When \( a < -1 \), the nature of the roots for the equation
$$
\left(a^{3}+1\right) x^{2}+\left(a^{2}+1\right) x-(a+1)=0
$$
is:
(A) Two negative roots.
(B) One positive root and one negative root, with the absolute value of the negative root being larger.
(C) One positive root and one negative root, with the absolute value of the negative root being smaller.
(D) No real roots.
assistant
<|begin_of_thought|>

Deep Thought is thinking for 7.5 million years.

<|end_of_thought|>

<|begin_of_solution|>


Given the quadratic equation:

\[
(a^3 + 1) x^2 + (a^2 + 1) x - (a + 1) = 0
\]

where \( a < -1 \).

1. **Identify the coefficients and analyze their signs:**

    - \( a^3 + 1 \)
    - \( a^2 + 1 \)
    - \( -(a+1) \)

2. **Signs of the coefficients under the condition \( a < -1 \):**

    - Since \( a < -1 \), we have \( a^3 < -1 \), therefore \( a^3 + 1 < 0 \).
    - Given \( a^2 \geq 0 \) and \( a^2 + 1 \geq 1 \), it follows that \( a^2 + 1 > 0 \).
    - Since \( a < -1 \), then \( a + 1 < 0 \).

3. **Discriminant (\(\Delta\)) of the quadratic equation:**

    The discriminant \(\Delta\) of a quadratic equation \( Ax^2 + Bx + C = 0 \) is given by:
    \[
    \Delta = B^2 - 4AC
    \]

    Here \( A = a^3 + 1 \), \( B = a^2 + 1 \), and \( C = -(a + 1) \). Thus, the discriminant is:
    \[
    \Delta = (a^2 + 1)^2 - 4(a^3 + 1)(-(a + 1))
    \]

4. **Simplify the discriminant:**

    Substituting \( A \), \( B \), and \( C \) into the discriminant formula:
    \[
    \Delta = (a^2 + 1)^2 + 4(a^3 + 1)(a + 1)
    \]

    Since \( a^3 + 1 < 0 \) and \( a + 1 < 0 \), their product is positive. Consequently, the discriminant \(\Delta > 0\), indicating:

    \[
    \Delta > 0
    \]
    
    Therefore, the quadratic equation has two distinct real roots.

5. **Determine the nature of the roots using Vieta's formulas:**

    According to Vieta's formulas:
    - The sum of the roots \( x_1 \) and \( x_2 \) of the equation \( Ax^2 + Bx + C = 0 \) is given by: 
      \[
      x_1 + x_2 = -\frac{B}{A}
      \]
    - The product of the roots is given by:
      \[
      x_1 x_2 = \frac{C}{A}
      \]

    For our equation:
    \[
    x_1 + x_2 = -\frac{a^2 + 1}{a^3 + 1}
    \]
    \[
    x_1 x_2 = \frac{-(a + 1)}{a^3 + 1}
    \]

6. **Sign of the roots' product:**

    Given \( x_1 x_2 = \frac{-(a + 1)}{a^3 + 1} \) and because \( a + 1 < 0 \) and \( a^3 + 1 < 0 \):
    \[
    x_1 x_2 = \frac{-(a + 1)}{a^3 + 1} < 0
    \]

    This implies that the roots \( x_1 \) and \( x_2 \) have opposite signs.

7. **Sum of the roots:**

    Given \( x_1 + x_2 = -\frac{a^2 + 1}{a^3 + 1} \) with \(a^2 + 1 > 0 \) and \( a^3 + 1 < 0 \):
    \[
    x_1 + x_2 = -\frac{a^2 + 1}{a^3 + 1} > 0
    \]

    This implies that the negative root in absolute value is smaller than the positive root.

### Conclusion:

Thus, the correct answer is:
\[
\boxed{C}
\]

<|end_of_solution|>