The sentence "Le medecin soigne _ malades" requires a definite article to complete the meaning: "The doctor treats the sick (people) / the patients."
The word malades is plural, so the required article is les, the plural definite article in French. The definite article is correct here because the sentence makes a general statement about what doctors do - they treat patients as a class. In French, general statements about entire categories use the definite article (le, la, les), unlike English where the article is often dropped.
The other options do not fit:
une is feminine singular - it does not agree with the plural malades.
la is feminine singular definite - also does not agree in number.
des (indefinite plural, meaning "some") would change the meaning to "the doctor treats some sick people," which weakens the general truth the sentence expresses.
