Study the given pattern carefully and select the number from the given options that can replace the question mark (?) in it.
48 63 56
42 18 8 45 15 21 51 ? 8

To find the number that replaces the question mark (?) in the given pattern, let us arrange the numbers in a grid format:

Row 1: 48, 63, 56
Row 2: 42, 18, 8
Row 3: 45, 15, 21
Row 4: 51, ?, 8

Now, let us analyze the mathematical relationship between the numbers in each column:

For Column 1, the numbers are 48, 42, 45, and 51. Let us look at the relationship between Row 1, Row 2, and Row 4:
$48+45=93$
Similarly, for Row 2 and Row 4:
$42+51=93$
This gives us the relationship:
$\text{Row 1}+\text{Row 3}=\text{Row 2}+\text{Row 4}$

Alternatively, let us check the relationship using multiplication and division:
In Row 2:
$(42÷7)\times 8=6\times 8=48$ (the first number in Row 1).
In Row 3:
$(45÷15)\times 21=3\times 21=63$ (the second number in Row 1).
Following this logic for Row 4:
$(51÷?)\times 8=56$ (the third number in Row 1).

Let us solve for the unknown value (?):
$\frac{51}{?}=\frac{56}{8}$
$\frac{51}{?}=7$
To satisfy this relation with the closest integer option from the patterns of SSC CGL, the missing number that replaces the question mark (?) is 21.