Answer:
The correct missing word is: "<em>al</em>".
Explanation:
In Spanish all nouns go with an article. When the noun is singular and masculine, as the word "<em>mercado</em>" (market) is, it is correct to use the article "<em>el</em>" before the noun.
Now, the sentence is presenting a situation in which some people go <em><u>to</u></em> the market every Tuesday. The preposition "<em>a</em>"<em> in its function for denoting place</em>, it is used in cases like this, in which it is pointed out a place. So, that would apparently leave us a phrase like this: "Ellos van <u><em>a el</em></u> mercado los martes", but is incorrect, for the preposition "<em>a</em>" next to the article "<em>el</em>" contract into one word: "<em>al</em>". This contraction only happens with the singular masculine nouns. THEREFORE, the correct answer is the word: <u>"</u><u><em>al</em></u><u>".</u>