Step 1: Understanding quantiles
Quantiles divide the dataset into equal parts. The second quantile (Q2) corresponds to the 50th percentile.
Step 2: Identifying Q2
The 50th percentile is also known as the median, because it represents the middle value of ordered data.
Step 3: Eliminating incorrect options
- Mode is the most frequent value, not the second quantile.
- Mean is the arithmetic average, not the median.
- Standard deviation measures dispersion, not a quantile.
\[
\boxed{\text{Therefore, the second quantile = Median.}}
\]