The duties of the senate changed thru the time as well as the way they were appointed. In the early ages of Rome, the senate was there to advise the king. During the Roman Republic the senate became more powerful. Although the senate could only make "decrees" and not laws, its decrees were generally obeyed. The senate also had control of the spending of the state money, situation that made the senate very powerful. Later, during the Roman Empire, the senate had less power and the real power was held by the emperor, who actually chose the member of the senate and set the requirements to be appointed in the senate. One of the most common and more important rules was that a member of the senate was required to have over 1 million sesterces (ancient Roman coin) in wealth. If they came into misfortune and lost their wealth, they were expected to resign.
In regards to the specific way in which the senate was formed, the answer to this question is:
C The Roman Senate was originally made up of only patricians, though some plebeians were eventually granted power as tribunes and even rose to the position of consul.