In the following command, what part of speech is 'se'? no se los ponga. reflexive pronoun indirect object pronoun direct object pronoun

The answer is Reflexive Pronoun

Explanation:

In Spanish there are reflexive verbs and reflexive pronouns. A reflexive verb indicates that the action is performed on the subject themselves, in other words, it indicates that someone or something is performing an action on or for itself. For example, the verb despertarse is reflexive, and means To wake oneself up. Reflexive verbs are formed by adding the reflexive pronoun "se" at the end of the verb in infinitive form. i.e. despertar-se (to wake oneself up), lavar-se(to clean oneself up) , levantar-se (to get oneself up), vestir-se (to get dressed), etc...

Now, when we conjugate one of these verbs with the different pronouns, we add the Reflexive Pronoun before the verb. The reflexive pronouns for each personal pronoun are:

Subject              Reflexive Pronoun

Yo                                 Me

Tu                                 Te

Él/Ella                           Se

Nosotros                      Nos

Usted/ Ustedes           Se                    

Ellos/Ellas                    Se

So in this case, the reflexive verb is ponerse (to put on), and if we conjugate it it would be

Yo me pongo

Tu te pones

Él/ella se pone

Nosotros nos ponemos

Usted se pone

Ustedes se ponen

Ellos/Ellas se ponen

Now, if we turn in into a negative command, we add "No" at the beginning,  

and the Indirect Object Pronoun "Los" to refer to the object they're talking about. "No se los ponga" would translate to "Don´t put them on". "Los" is referring to whatever object the person "can´t" put on.