The word from the list that best fits the definition "Ellos van mercado todos los martes" is "al."
Explanation:
1. "Ellos van al mercado todos los martes" translates to "They go to the market every Tuesday."
2. In Spanish, the preposition "a" is used before a specific place or destination.
3. The combination of "a" + "el" becomes "al" through contraction.
4. Therefore, in this sentence, "al" is the correct choice as it indicates the direction or destination where they go, which is "to the market."
Therefore, the word "al" from the list is the best fit for the given sentence.