Aheroin addict went through the following treatment to stop his drug abuse: a doctor gradually replaced his daily heroin dosage with the legal drug methadone. what kind of treatment is this

Methadone has been used since 1960 as a substitute for opiates among heroin users.
(Substitution treatment)
It is primarily used as replacement therapy in heroin addicts: methadone has properties comparable to heroin, but it has a more favorable pharmacological profile. This transfer of dependence can stabilize the patience and facilities rehabilitation. 

Can you list the drugs that you want answered for? i think maybe your answer could be a or b. but i’ll never know till you post the rest of this question.