A culture containing 21 bacteria doubles every day. Which of the following formulas represents the number of bacteria, $p$, in the culture after $t$ days? \[ \begin{array}{l} p=21(2)^{t} \\ p=21(1.2)^{t} \\ p=21+2 t \\ p=42 t \\ p=21^{2 t} \end{array} \]

Step 1 ：The problem states that the bacteria doubles every day. This means that the number of bacteria is multiplied by 2 each day.

Step 2 ：Therefore, the formula that represents this situation is \(p=21(2)^{t}\), where \(p\) is the number of bacteria, \(21\) is the initial number of bacteria, \(2\) is the rate of growth, and \(t\) is the number of days.

Step 3 ：Final Answer: The formula that represents the number of bacteria, \(p\), in the culture after \(t\) days is \(\boxed{p=21(2)^{t}}\).