I know what the answer is but please show work on how to solve it (HANDWRITTEN AND NOT USING EXCEL)

The index model has been estimated for stocks A and B with the following results: IA - If = 0.01 + 0.8 (IM - rf) + ea. IB - rf = 0.02 + 1.2 (rm - rf)+ eB, o(ea) = 0.20 = OM = 0.20, (EB) = 0.10. = The standard deviation for stock A is A) B) C) D) E) 0.0656 0.0676 0.2561 0.2600 none of the above