Answer:
None of the options are correct.
The correct answer lies between 2 percent through to 15.6 percent.
Step-by-step explanation:
We have been given enough data or parameters or information which is going to help in solving the problem effectively and efficiently; 
So, we have the following main points from the question/problem given above: 
=> " One stock has an expected return 
= 12% and a standard deviation of 24%."
=> The other has an expected return of 8% and a standard deviation of 20%."
=> "50% of your investment went toward each of the two stocks." 
=> "If the two stocks are negatively correlated"
Hence, the most feasible standard deviation of the portfolio can be calculated as below: 
(1). For stock portfolio One , we have 
 (0.5 × 20/100 ) 
(2). For stock polio two, we have; 
( 0.5 × 24/100) 
(3). 2 × 0 5 × (20/100) × 0.5 × (24/100) × correlation. 
Now, take the square of (1) and (2) above and then the square root of (1), (2) and (3); and add (1), (2) and (3) together to give the standard deviation; 
√[0.0244 + 0.024 × b]. ---------------------------(****). 
Where b = correlation. 
NOTE : "We are to assume that stocks are negatively correlated, thus, the correlation, bvalue between -1 and 0.
Thus, slotting in the correlation values into equation (****). 
=> So, between 2 percent through to 15.6 percent.