Final answer:
The molecular geometry of HCN is linear, as it has 2 electron domains comprising a single bond and a triple bond, which are arranged 180 degrees apart to minimize repulsion.
Step-by-step explanation:
The molecular geometry of HCN (hydrocyanic acid) can be determined by considering its electron group geometry. HCN has a total of 2 electron domains: one from the H-C single bond and the other from the C≡N triple bond. According to the VSEPR theory, the optimal arrangement of electron domains to minimize repulsion is to place them opposite each other, or at a 180° angle. This leads to a linear molecular geometry. Therefore, the correct answer to the question is A. Linear.