To find the cost of each Notebook \( x \), we start by setting up the equation based on Mia's purchases.
Mia buys:
- 5 Notebooks at \( x \) dollars each, for a total of \( 5x \) dollars.
- 5 pencils at $0.75 each, for a total of \( 5 \times 0.75 = 3.75 \) dollars.
The total cost for everything is given as $23.50. Therefore, we can write the equation:
\[ 5x + 3.75 = 23.50 \]
To isolate \( 5x \), we subtract \( 3.75 \) from both sides:
\[ 5x = 23.50 - 3.75 \]
Calculating the right side:
\[ 5x = 19.75 \]
Now, we solve for \( x \) by dividing both sides by 5:
\[ x = \frac{19.75}{5} = 3.95 \]
Thus, the cost of each Notebook is:
\[ \boxed{3.95} \]
So the correct answer is D. $3.95.