In the following data set, what is the most frequently occuring number?

$21$

$33$

$21$

$12$

$33$

$21$

$32$

The correct option is B $21$
Step 1:

To find the frequency of each number of the given data set, we arrange the numbers in ascending order.

$12$

$21$

$21$

$21$

$32$

$33$

$33$

Step 2:

Now, we create a frequency table to determine the frequency of each data points or number of time(s) each number appears in the given data set.
Frequency Table

Number	Frequency
$12$	$1$
$\underset{–}{21}$	$\underset{–}{3}$
$32$	$1$
$33$	$2$

From the above frequency table, it is clear that the number $21$ appears the most number of times.

$\underset{–}{\text{Note:}}$ The most frequently occuring value or thing in a data set is called $\text{mode}$.

Hence, the mode of the given data set is $21$.