Choose the number that is odd or contextually different from the rest of the group:

Let's look at the mathematical properties of the numbers in the group:
* $12$ is completely divisible by $3$ ($12÷3=4$).
* $21$ is completely divisible by $3$ ($21÷3=7$).
* $24$ is completely divisible by $3$ ($24÷3=8$).
* $35$ is not divisible by $3$.

Since $35$ is the only number in the set that is not a multiple of $3$, it is the odd one out.