Translation
The Doctor is giving an injection to the guy/boy
<h2>Correct answer:</h2><h3>hace tres días que yo nado en el océano
</h3>
If you want to indicate that you swam in the ocean <em>three days ago,</em> you would indicate this by saying <em>hace tres días que yo nado en el océano.</em> Since this sentence is given in the simple present we need to conjugate the verb nadar using this tense and matching the first person singular, so this conjugation is nado.
