The total resistance in the circuit is the sum of the internal resistance and the resistance of the resistor.
The time constant \( \tau \) is given by:
\[
\tau = R \cdot C
\]
where \( R \) is the total resistance and \( C \) is the capacitance.
The time constant for this circuit is:
\[
\tau = 0.5 \, \Omega \times 5 \, \mu F = 2.5 \, \mu \text{s}
\]
The charge on the capacitor is given by the formula:
\[
Q = C \cdot V
\]
where \( C = 5 \, \mu F \) and the voltage \( V = 2.5 \, \text{V} \).
Therefore, the charge on the capacitor is:
\[
Q = 5 \, \mu F \times 2.5 \, \text{V} = 10 \, \mu C
\]
Thus, the correct answer is (c).