Final answer:
The rock that misses Frodo and Sam does not correspond with any typical volcanic projectiles mentioned and seems to be part of a fictional scenario. Therefore, the answer is 'none of the above'.
Step-by-step explanation:
The rock described in the scenario is not a type of volcanic projectile often associated with terrestrial volcanoes, such as a bomb from a Hawaiian style volcano or a xenolith, nor is it related to a mud volcano or a mid-ocean ridge. Given the context of Frodo and Sam, characters from fiction standing by Mount Doom, a fictional volcano, and the description of the rock missing them, it seems most likely that this is a creative illustration rather than a scientific question. However, if we consider the options available, it would be more accurate to refer to the rock as 'none of the above' since it seems to be a fictional scenario and the rock does not fit the typical characteristics of the given choices.