Tanisha, a 32 year old woman diagnosed with OCD, has a recurrent thought that she is going to stab her husband with a knife. This thought is very distressing to her and she has no intention to stab her husband. Each time she has this thought, she prays and repeats the same prayer until the anxiety subsides. In this case, praying is

a mental ritual 

a decontamination behavior 

a checking behavior 

an obsession