The sentence Il dort depuis ___ heures means "He has been sleeping for ___ hours." The blank requires a word that can precede the plural noun heures (hours) and make grammatical and semantic sense with depuis (since/for).
The correct answer is deux (two). Deux is a cardinal number that specifies a definite duration: "He has been sleeping for two hours." It agrees with the plural noun heures.
The other options present problems:
des (some/several) could form the expression depuis des heures (for hours), but deux is the more precise and standard completion being tested here as a numerical determiner
le is a masculine singular definite article and cannot precede the feminine plural heures
une is a feminine singular indefinite article. It would require the singular heure (not heures), so it does not agree with the plural form given
The complete sentence is: Il dort depuis deux heures - He has been sleeping for two hours.
