Answer
If the mass of a radioactive sample is doubled, the activity increases.
Work Step by Step
We can write an expression for the decay rate:
$R = \lambda~N = \frac{N~ln(2)}{T_{1/2}}$
$N$ is the number of radioactive atoms
$T_{1/2}$ is the half-life
If the mass is doubled, then the value of $N$ doubles. Since $T_{1/2}$ does not depend on the mass, $R$ also doubles when the mass is doubled.
Therefore, if the mass of a radioactive sample is doubled, the activity increases.
