Learn Dutch: Welke partij wint de verkiezing dit jaar? - Which party is winning the election this year?

Because partij is a de-word (de partij), you have to use welke. The word welk is only used with het-words, like welk boek (which book).

Why does the sentence use the present tense wint if the election is later dit jaar?

In Dutch, you can often use the simple present tense to talk about the future, as long as you include a clear time phrase like dit jaar (this year). You could also say gaat winnen or zal winnen, but using the present tense is very common and natural.

Why is it dit jaar instead of deze jaar?

The word for "this" depends on the gender of the noun. Because jaar is a het-word (het jaar), you must use dit. You only use deze for de-words and for plurals.

Is it also correct to use the plural de verkiezingen here?

Yes! Just like in English, Dutch speakers often use the plural de verkiezingen (the elections) to refer to a single general voting event. Both the singular and plural forms work perfectly in this context.