Answer
1.729.
Work Step by Step
The dot plot is roughly bell-shaped, although the sample size is less than 30 (it is 20), hence the sample data is roughly normally distributed, hence $\alpha=1-0.9=0.1.$ $\sigma$ is not known, hence we us the t-distribution with $df=sample \ size-1=20-1=19$ in the table. $t_{\alpha/2}=t_{0.05}=1.729.$
