Amensalism is a relationship between two population in which only one of them suffers, and the other one is not affected. Here, a moose is not affected, it just passes by, it is not negatively or positively affected. However, the meadow grass is negatively affected because it is often trampled. So, we have the not affected moose and the meadow grass which suffers. Therefore, this is example of amensalism.
In commensalism one of those species would benefit, and the other one would not be not affected. In parasitism one of those species would benefit at the expense of the host organism. Also, this is not interspecific competition since interspecific means inside the species, and the moose and the meadow grass are not the same species.