What are unit of activity of radioactive elements?

Which of the following is the unit of activity of a radiactive element?

Write the SI unit of activity of a radioactive nuclide.

Which one of following is the unit of activity of a radioactive source ?

The activity of a radioactive sample

What is meant by activity of a radioactive sunstance? Write its SI unit.

The nuclear activity of a radioactive element becomes (1/8)^(th) of its initial value in 30 years. The half-life of radioactive element is ________ years.