Magnetic excitation spectra of Ba(Fe 0.9 Co 0.1 ) 2 As 2 obtained for aligned crystals with the superconducting transition temperature T c 23 K are presented after an overview of results of our preceding studies on the symmetry of the order parameter . Although many researchers have reported that their experimental results are consistently explained by the so-called S symmetry, and believe that the spin-fluctuation is relevant to the superconducting mechanism of Fe pnictide superconductors. However, the situation is not so simple, because almost all these data can be understood by the S ++ -symmetry, too. Furthermore, as we first pointed out and discussed in detail, the rates of the T c suppression by nonmagnetic impurities can hardly be explained by the S symmetry. Here, we show our own data of the magnetic excitation spectra of Ba(Fe 0.9 Co 0.1 ) 2 As 2 , and discuss which one of the S and S ++ -symmetries can better describe the characteristics of the peak observed there. These processes are important, because the S and S ++ symmetries are connected with the spin-fluctuation mechanism and a newly proposed one related to the orbital degrees of freedom of the systems, respectively.