Answer and Explanation:
The calculations of the stock return for the missing year is shown below: 
 a. Let us assume the fifth year stock return be x 
As we know that 
Average rate of return = Total returns ÷ number of years 
 0.12 = (0.1 - 0.11 + 0.21 + 0.22 + x) ÷ 5 
 So after solving this, the x is 14%
 b. Now the standard deviation of the stock return is presented in the excel spreadsheet 
 The standard deviation is 13.40%