To find the inequality that represents all the possible lengths of the base \( b \) of the rectangle, we start by using the formula for the area of a rectangle, which is given by:
\[ \text{Area} = \text{height} \times \text{base} \]
Given that the height is 4 units and the area can be up to 48 square units, we can write the inequality:
\[ 4 \times b \leq 48 \]
To isolate \( b \), we divide both sides of the inequality by 4:
\[ b \leq \frac{48}{4} \]
Calculating the right side gives:
\[ b \leq 12 \]
Thus, the correct inequality that represents all the possible lengths of the bases \( b \) that Saritha can use is:
C: \( b \leq 12 \)