Answer
1.35.
Work Step by Step
The ray enters the prism with a zero degree incident angle (normal) and is not bent.
The incident angle of $60^{\circ}$ at the glass-water interface is the critical angle.
Total internal reflection occurs when the angle of incidence is greater than or equal to the critical angle.
At the critical angle, Snell’s law gives $n_{glass}sin\theta_{crit}= n_{liquid}sin90^{\circ}$.
$$ 1.56sin60.0^{\circ}= n_{liquid} (1)$$
$$ n_{liquid}=1.56\frac{ sin60.0^{\circ}}{1} = 1.35$$
