Tony has thoughts about hating his younger brother and wishing he would die. He becomes very anxious about these thoughts because he has developed the idea that if anything really happened to his brother, it would be his fault. For no explainable reason, tony starts mentally counting by odd numbers each time he walks past his brother's room and discovers that this activity makes him less anxious. Tony's behavior can be described as

Tony hates his brother so much that he has started to think that if anything bad happens to his brother, he would be the one responsible for that. So whenever he passes by his brother's room, he starts to count odd numbers, this helps him lessen his anxiety. Tony's this behavior describes as the Mental compulsion.

Compulsions are behaviors that are repeated again and again to reduce the anxiety that is related to some obsession, and obsessions are unreasonable, unrelated thoughts or fears that make a person anxious. So to reduce the obsessions, a person may act compulsive. He tries to find out ways to reduce his anxiety, just like in the given question, Tony starts to count odd number whenever he passes by his brother room to reduce his fears and unreasonable thoughts.