This question concerns the conjugation of the verb nadar.
Verbs are normally conjugated with an -amos affix to the verb for nosotros, the first person plural form.
Naderemos is a nonsense answer, as this isn't a conjugation found anywhere in a conjugation chart for nadar.
Nadan is a conjugation for ellos/ellas/ustedes, or third person plural form.
Namos a nadar is also a nonsense answer, as namos doesn't exist as a word in the Spanish language, and the correct way to say this would be "Vamos a nadar."
The answer is nadamos, as that is the correct conjugation for nosotros.