What is the decay constant of 23892u whose half-life is 4.5×109year?

Answer is: decay constant for uranium-238 is 1,54·10⁻¹⁰ 1/y.
The half-life for the radioactive decay of U-238 is 4.5 billion years and is independent of initial concentration.
t1/2 = 4.5·10⁹ y.
λ = 0,693 ÷ t1/2.
λ= 0,693 ÷ 4.5·10⁹ y.
λ = 1,54·10⁻¹⁰ 1/y.
λ - decay constant.
Decay constant is proportionality between the size of a number of radioactive atoms (in this example uranium) and the rate at which the number of radioactive uranium atoms decreases because of radioactive decay. 

The answer is a) carbon-12