\begin{align*}
5^{21} \cdot 4^{11} &= 2\cdot 10^p \\
5^{21} \cdot 2^{22} &= 2 \cdot (2\cdot 5)^p \\
5^{21} \cdot 2^{22} &= 2 \cdot 2^p \cdot 5^p \\
5^{21} \cdot 2^{22} &= 2^{p+1} \cdot 5^p
\end{align*}
Now, we equate the exponents an get
\begin{align*}
p+1 &= 22\\
p &= 21
\end{align*}
Therefore, B
