To estimate the price of one share of stock after 20 years using the equation \( y = 200 - 8x \), we need to substitute \( x = 20 \) into the equation:
\[ y = 200 - 8(20) \]
Calculating \( 8(20) \):
\[ 8(20) = 160 \]
Now substitute back into the equation:
\[ y = 200 - 160 \]
\[ y = 40 \]
Thus, the best estimate of the price of one share of stock after 20 years is $40.00.
The answer is B. $40.00.