The fraction f of radioactive material that has decayed in time t, varies with time t. The correct variation is given by the curve

Number of atoms undecayed N=N0e-λtNumber of atoms decayed =N0-N=N0(1-e-λt)⇒ Decayed fraction f=N0-NN0=1-e-λti.e., fraction will rise up to 1, following exponential path as shown in graph (B).