What is the decrease in the SDLP's seats from 1998 to 2017?

To determine the decrease in the SDLP's seats from 1998 to 2017, we can look at the seat counts during those years. In 1998, the SDLP won 24 seats, while in 2017, their representation dropped to 12 seats. The calculation of the decrease is straightforward: subtract the number of seats in 2017 from those in 1998 (24 - 12), which results in a decrease of 12 seats.

Therefore, selecting the choice that accurately states this decrease (24–12) is correct. This reflects a significant reduction in the SDLP's parliamentary presence over this period, illustrating a broader trend of declining support for the party in Northern Ireland politics. The other options present incorrect figures that do not align with the documented seat counts from those election years.