They choose it randomly
The priest leads the Eucharistic prayers, the people at the Mass , as well as the priest off the sacrifice up to God.
There are many prayers prayed by the priest during the Eucharist. I'm thinking that you are asking about a specific one, but this is just a guess. The Canon or Eucharistic Prayer is the central portion of the Eucharistic liturgy, it has eight parts: Preface, Acclamation, Epiclesis, Consecration, Anamnesis, Oblation, Intercessions, and Doxology.
the priest or eucharistic minister
A bishop, priest, deacon, or Eucharistic minister.
It occurs during the Eucharistic Prayer right after the consecrationof the wine by the priest, after the priest says "..do this in memory of me".
These are led by the priest in Church. Lay people do not lead prayers in Church. In some instances, such as during Vigil services, the priest will start the prayers, and then lay people continue to read them. But where a priest is present with lay people, only he can lead the prayers.
the priest
That is a very interesting question. Do you mean Canon? This question can have various answers depending on what you mean! The Canon of the Mass is the Eucharistic Prayer (What the priest says after the Holy, Holy Holy up to and including the through him with him and in him"). It includes the consecration (saying "this is my body" and "this is the cup of my blood . . . " ) epiclesis (invocation of the holy ghost) and other parts. The Roman Canon is one of the Eucharistic Prayers that the priest can say. Before Vatican II it was the only Eucharistic Prayer/Canon used at Mass. Now it is Eucharistic Prayer I (there are three others I believe the priest can use now after Vatican II). Canon Law is the legal system used by the Catholic Church to govern itself. A Canon can also be a priest who is (or was) involved in the administration of a cathedral or collegiate body.
A priest, deacon or Eucharistic minister may apply ashes on Ash Wednesday
Catholics do not TAKE Holy Communion, they may only RECEIVE it from a priest or a licensed Eucharistic Minister.
Once the Host has been consecrated by the priest, it becomes the Body of Christ. After consecration, the Eucharist is administered by either the priest, the deacon, or trained Eucharistic Ministers.
Islamic priest are called imam. and they lead prayers they also perich