Let $p$ a prime number and $d$ a divisor of $p-1$. Find the product of elements in $\mathbb Z_p$ with order $d$. ($\mod p$).
(10 points)
1. **Identify the elements in $\mathbb{Z}_p$ with order $d$:**
   Let $p$ be a prime number and $d$ a divisor of $p-1$. The elements in $\mathbb{Z}_p$ with order $d$ are those elements $x \in \mathbb{Z}_p$ such that the smallest positive integer $k$ for which $x^k \equiv 1 \pmod{p}$ is $d$. 

2. **Properties of elements in $\mathbb{Z}_p$:**
   Since $p$ is prime, $\mathbb{Z}_p^\times = \{1, 2, \ldots, p-1\}$ forms a cyclic group of order $p-1$. This means that for any $x \in \mathbb{Z}_p^\times$, there exists an integer $k$ such that $x^k \equiv 1 \pmod{p}$, and the smallest such $k$ is called the order of $x$.

3. **Inverse elements and their orders:**
   For any $x \in \mathbb{Z}_p^\times$, there exists an inverse $y \in \mathbb{Z}_p^\times$ such that $xy \equiv 1 \pmod{p}$. If $x$ has order $d$, then $y$ also has order $d$. This is because if $x^d \equiv 1 \pmod{p}$, then $(xy)^d \equiv 1 \pmod{p}$ implies $y^d \equiv 1 \pmod{p}$.

4. **Product of elements with order $d$:**
   Let $S$ be the set of elements in $\mathbb{Z}_p^\times$ with order $d$. Since $S$ is closed under taking inverses and $d \neq 1$ or $2$, we can pair up the elements of $S$ into disjoint pairs $(x_i, y_i)$ such that $x_i y_i \equiv 1 \pmod{p}$. 

5. **Calculate the product:**
   \[
   \prod_{x \in S} x = \prod_{i=1}^{\frac{|S|}{2}} x_i y_i \equiv \prod_{i=1}^{\frac{|S|}{2}} 1 \equiv 1 \pmod{p}
   \]
   This shows that the product of all elements in $S$ is congruent to $1 \pmod{p}$.

6. **Special cases for $d = 1$ and $d = 2$:**
   - If $d = 1$, then $S = \{1\}$, and $\prod_{x \in S} x = 1$.
   - If $d = 2$, then $S = \{p-1\}$, and $\prod_{x \in S} x = p-1 \equiv -1 \pmod{p}$.

7. **Conclusion:**
   For $d \neq 2$, the product of elements in $\mathbb{Z}_p$ with order $d$ is $1 \pmod{p}$. For $d = 2$, the product is $-1 \pmod{p}$.

The final answer is $\boxed{1 \pmod{p}}$ for $d \neq 2$ and $\boxed{-1 \pmod{p}}$ for $d = 2$.