Final answer:
The correct word is 'el'.
Step-by-step explanation:
The correct word from the options that best fits the definition 'El muchacho sabe (knows) el secreto' is 'el'.
'El' is a masculine singular definite article in Spanish and it corresponds to the word 'the' in English. In this sentence, it is used to refer to a specific secret that the boy knows.
For example, we could say 'El muchacho sabe el secreto de su amigo' which translates to 'The boy knows his friend's secret'.
Learn more about Spanish grammar