Copy field from Account to Opportunity (is a picklist)

I have a field on the Acccount that is called Account_Tier, this is a picklist with the following options: Tier I, Tier II, Tier II. I need to just make an easy formula field to copy this field and the value over to the Opportunity. Ccan you please help me out with same senerio as when I try to create an formula feild on Oppurtunity, cant find account feilds to Insert. Thanks

Or any aleter name solution just need the same values of related accout tier on Opp for reporting