Answer
12 V.
Work Step by Step
Use the transformer relationship.
$$\frac{primary \; voltage}{\# \; of \; primary \; turns} = \frac{secondary \; voltage}{\# \; of \; secondary \; turns}$$
$$\frac{120 V}{100 turns} = \frac{secondary \; voltage}{10 turns}$$
The secondary voltage is 12 V.