To calculate the number of primary turns needed for a transformer, you will need the primary voltage. The relationship between the number of turns in the primary and secondary coils and their respective voltages is given by the transformer equation:
\[ \frac{V_p}{V_s} = \frac{N_p}{N_s} \]
where \( V_p \) is the primary voltage, \( V_s \) is the secondary voltage, \( N_p \) is the number of primary turns, and \( N_s \) is the number of secondary turns.
In your case, you already know the number of secondary turns (10 turns) and the secondary voltage (50 V). Thus, to find the number of primary turns, you would need to know the primary voltage. Therefore, the correct answer is:
the primary voltage.