Stasimon in Greek Tragedy is a stationary song composed by strophes and antistrophes and performed by the chorus in the orchestra.
According with Aristoteles each choral song is divided into two parts, first the parodos and then the stasimon.
The basic structure of Greek Tragedy can be seen as one of acts and act dividing songs. They usually begin with a prologue that it´s the part of the play wich preceeds the entrance of the chorus (parodos) and it´s divided normally in two scenes; one of these scenes always contains a monologue spoken by a character of the play and is addressed to the audience.
Right after the prologue or even in some plays during it takes place the entrance of the chorus knew by the name of parodos, they also present the song they will be singing. And then starts the first episode wich is a conjunction of several scenes of the play and after the first episode comes the first stasimon (A). It´s a extended song performed by the chorus who already had made its entrance and it is almos always made in an antistrophic form. As the play advances the Stasimon tend to ger shorter.
Finally after the final stasimon is sung came the exodos to put an end to the play.
Just to clarify a lilttle I will repeat that the correct answer according to what I have written above is option A.
I hope that the answer help you. If there is any doubt I will not be bothered to try to explain it better. Regards