"He knows me."
Translation:El mă cunoaște pe mine.
It depends on the context. We say that, in general, the accented/stressed pronoun is optional, but there are situations where it is mandatory.
Here are two examples to illustrate how "pe mine" moves the emphasis in the economy of the sentence. Please note that the English answer is identical in both examples ("He knows me.").
Example 1 - "pe mine" should not be used
Q: Te cunoaște el? = Does he know you?
A: (Da,) mă cunoaște. = (Yes,) he knows me. --- We don't answer this question with "Mă cunoaște pe mine." In "Mă cunoaște." the verb (the action) is emphasized.
Example 2 - "pe mine" has to be used
Q: Pe cine cunoaște el? = Whom does he know?
A: Mă cunoaște pe mine. = He knows me. --- We don't answer this question with "Mă cunoaște." In "Mă cunoaște pe mine." the direct object is emphasized.
In English, we use intonation to accentuate "knows" in Example 1, and "me" in Example 2.
As Duo doesn't offer any context, both answers should be accepted.