There are $100$ people in a room. Some are [i]wise[/i] and some are [i]optimists[/i].

$\quad \bullet~$ A [i]wise[/i] person can look at someone and know if they are wise or if they are an optimist.
$\quad \bullet~$ An [i]optimist[/i] thinks everyone is wise (including themselves).

Everyone in the room writes down what they think is the number of wise people in the room. What is the smallest possible value for the average?
$$
\mathrm a. ~ 10\qquad \mathrm b.~25\qquad \mathrm c. ~50 \qquad \mathrm d. ~75 \qquad \mathrm e. ~100
$$
1. Let \( x \) be the number of wise people in the room. Therefore, the number of optimists is \( 100 - x \).

2. Each wise person knows exactly how many wise people are in the room, so each wise person writes down \( x \).

3. Each optimist thinks everyone is wise, so each optimist writes down \( 100 \).

4. To find the average number of wise people written down, we calculate the total sum of the numbers written down by all individuals and then divide by the total number of people (100).

5. The total sum of the numbers written down is:
   \[
   x \cdot x + (100 - x) \cdot 100
   \]
   This is because \( x \) wise people each write \( x \), and \( 100 - x \) optimists each write \( 100 \).

6. Simplifying the expression:
   \[
   x^2 + 100(100 - x)
   \]
   \[
   = x^2 + 10000 - 100x
   \]

7. The average number of wise people written down is:
   \[
   \frac{x^2 + 10000 - 100x}{100}
   \]

8. To find the minimum value of this average, we can rewrite the expression:
   \[
   \frac{x^2 + 10000 - 100x}{100} = \frac{x^2 - 100x + 10000}{100}
   \]

9. Completing the square for the quadratic expression \( x^2 - 100x + 10000 \):
   \[
   x^2 - 100x + 2500 + 7500 = (x - 50)^2 + 7500
   \]

10. Substituting back into the average expression:
    \[
    \frac{(x - 50)^2 + 7500}{100} = \frac{(x - 50)^2}{100} + 75
    \]

11. The term \(\frac{(x - 50)^2}{100}\) is always non-negative, and its minimum value is 0 when \( x = 50 \).

12. Therefore, the minimum value of the average is:
    \[
    75 + \frac{(50 - 50)^2}{100} = 75
    \]

Conclusion:
\[
\boxed{75}
\]