What is the least number of weighings needed to determine the sum of weights of $13$ watermelons such that exactly two watermelons should be weighed in each weigh?

$ 
\textbf{a)}\ 7
\qquad\textbf{b)}\ 8
\qquad\textbf{c)}\ 9
\qquad\textbf{d)}\ 10
\qquad\textbf{e)}\ 11
$
1. **Understanding the problem**: We need to determine the sum of the weights of 13 watermelons using the least number of weighings, where each weighing involves exactly two watermelons.

2. **Initial analysis**: Let's denote the weights of the 13 watermelons as \( x_1, x_2, \ldots, x_{13} \). We need to find the sum \( S = x_1 + x_2 + \cdots + x_{13} \).

3. **Weighing pairs**: Each weighing gives us the sum of the weights of two watermelons. For example, if we weigh \( x_1 \) and \( x_2 \), we get \( x_1 + x_2 \).

4. **Minimum number of weighings**: To determine the sum of the weights of all 13 watermelons, we need to set up a system of equations that allows us to solve for \( S \). Each equation will be of the form \( x_i + x_j = w_{ij} \), where \( w_{ij} \) is the result of the weighing.

5. **System of equations**: We need at least 12 independent equations to solve for the 13 variables (weights of the watermelons). However, since we are interested in the sum \( S \), we can use fewer weighings if we can cleverly combine the equations.

6. **Example with 3 watermelons**: Consider the case with 3 watermelons:
   - Weigh \( x_1 \) and \( x_2 \): \( x_1 + x_2 = a \)
   - Weigh \( x_2 \) and \( x_3 \): \( x_2 + x_3 = b \)
   - Weigh \( x_3 \) and \( x_1 \): \( x_3 + x_1 = c \)
   - Adding these equations: \( (x_1 + x_2) + (x_2 + x_3) + (x_3 + x_1) = a + b + c \)
   - Simplifying: \( 2(x_1 + x_2 + x_3) = a + b + c \)
   - Therefore, \( x_1 + x_2 + x_3 = \frac{a + b + c}{2} \)

7. **Generalizing to 13 watermelons**: We can use a similar approach for 13 watermelons. We need to find a way to weigh pairs such that we can determine the sum \( S \).

8. **Weighing strategy**: We can weigh the first 3 watermelons as in the example above, which requires 3 weighings. For the remaining 10 watermelons, we can use 5 weighings (each weighing 2 watermelons).

9. **Total number of weighings**: 
   - Weigh the first 3 watermelons: 3 weighings
   - Weigh the remaining 10 watermelons: 5 weighings
   - Total weighings: \( 3 + 5 = 8 \)

10. **Conclusion**: The least number of weighings needed to determine the sum of the weights of 13 watermelons, with each weighing involving exactly two watermelons, is 8.

The final answer is \( \boxed{8} \)