Based on the experience from Prototype Imaging Spectrograph for Coronagraphic Exoplanet Studies (PISCES) for WFIRST, we have moved to the flight instrument design phase. The flight instrument is similar to PISCES, but there are important changes to its design as our requirements have evolved. Beginning with the science and system requirements, we discuss a number of critical trade-offs. Most significantly there are trades in the type of IFS, lenslet array shape and layout, detector sampling, and accommodating the larger Field Of View (FOV) and wider wavelength band for a potential Starshade. Finally, the traditional geometric optical design is also investigated and traded. We compare a reflective versus refractive design, and the telecentricity of the relay. The relay before the lenslet array controls the chief angle distribution on the lenslet array. Our previous paper1 has addressed how the relay design combined with lenslet array/pinhole mask can further suppress the residual star light and increase the contrast. Highlighting all of these design trades, we present the phase A IFS optical design for the WFIRST coronagraph instrument.