Synthetic transmission spectra for HD 191939 d. Models assuming a clear atmosphere with solar abundances (solid red line), a clear atmosphere with metallicity enhanced by a factor of 100 (blue dashed line), and a hazy atmosphere with solar abundances (green dotted line). Simulated measurements with error bars are shown for the observation of one transit with JWST NIRISS-SOSS, NIRSpec-G395M, and MIRI-LRS configurations.