This is a good example of the usage of the Inmediate Future Tense in Spanish.
The Inmediate Future Tense (also called Near Future Tense) is formed with the personal forms of the verb ir (to go) + the preposition a followed by the infinitive of the main verb. As follows:
This is a verbal periphrasis that allows the speaker to express in the present a statement with a sense of the future.
Note this tense is used in Spanish instead of the simple future to express a planned action or something that will happen soon, to express future scenarios and intentions.
So, in this same context we have a conversation between Ricardo and Alex, where Ricardo asks Alex about his plans for the next month:
Ricardo: ¿Qué planes tienes para el mes que viene?
Ricardo: What plans do you have for next month?
Then, Alex answers using the Inmediate Future Tense with the construction explained above, taking into account the verb ir (to go) is conjugated with the 1st person singular yo (I):
Alex: I am going to work as a volunteer for a social organization
The options vas (you are going), vamos (we are going) and va (he is going) are incorrect because do not match with the subject yo (I).
