Step 1: Quantile Definition
Quantiles partition a dataset into uniform segments. The second quantile (Q2) aligns with the 50th percentile.
Step 2: Q2 Identification
The 50th percentile is equivalently termed the median, signifying the central value in sorted data.
Step 3: Exclusion of Invalid Choices
- Mode: Represents the most frequent value, not the second quantile.
- Mean: Constitutes the arithmetic average, not the median.
- Standard Deviation: Measures data spread, not a quantile.
\[\boxed{\text{Consequently, the second quantile equals the Median.}}\]