In the Köppen system, the second letter s means a dry summer.
The code s describes a seasonal precipitation pattern, especially in temperate C and continental D climates. In the standard definition, the wettest winter month receives at least three times as much precipitation as the driest summer month. A climate such as Csa is therefore temperate, dry-summer, and hot-summer—typical of many Mediterranean regions.
The other common second-letter codes are w for dry winter and f for no distinct dry season. These letters follow the first group letter: C, for example, identifies a temperate climate, while s supplies its rainfall seasonality. The third letter then adds information about summer warmth in C and D climates.
A useful nuance is that s can also appear in the tropical code As, meaning tropical savanna with a dry summer. That does not make the quiz answer ambiguous: s is still the letter carrying the dry-summer meaning. The m code means monsoon in tropical classifications, not dry summer.