The work done on a charge is given by the equation:
\[
W = F \cdot d = qE \cdot d
\]
where \( W \) is the work done, \( q \) is the charge, \( E \) is the electric field, and \( d \) is the displacement. Using the given electric field \( E_0 \), the work done is:
\[
W = q \cdot E_0 \cdot 12
\]
Thus, the energy of the charge at \( x = 21 \) is given by \( 12qdE_0 \), where \( d \) is the displacement. This represents the total energy transferred to the charge as it moves in the electric field.
Hence, the correct answer is (c).