Liver fibrosis is fundamentally about excess collagen laid down by activated hepatic stellate cells, so the right answer is the stain that is specific for collagen. $Masson\ trichrome$ uses an acid dye sequence in which collagen retains aniline blue (or light green), giving fibrous septa a striking blue-green colour against red muscle and cytoplasm; this makes it the workhorse for grading hepatic fibrosis and confirming cirrhosis. The distractors target completely different targets: PAS marks glycogen and basement membranes and is used (with diastase) for alpha-1-antitrypsin globules, GMS is a silver method for fungi and Pneumocystis, and Gram stain separates bacteria into positive and negative groups. Since only one of these visualises collagen, the choice is unambiguous.
\[\boxed{\text{Masson trichrome}}\]