The correct answer is: [A]:  "Nade cerca de la costa."
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Step-by-step explanation:
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This is a short dialog.
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Note:
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Given:
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"Salvavidas: Señor, por favor. ___(blank)________________ " .
"Sr. García:  Está bien. "
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 →  "Among the answer choices provided, choose the best answer choice /phrase/ that belongs in the blank."
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The correct answer is:  [A]:  "Nade cerca de la costa."
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Note:
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"Salvavidas: Señor, por favor.  Nade cerca de la costa  " .
"Sr. García: Está bien. "
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Step-by-step explanation:
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The dialog begins with a "lifeguard" speaking. We know this because the "lifeguard" is identified by "occupation" rather than actual name; that is, the lifeguard is referred to as: "Salvavidas"; which is the Spanish word for "lifeguard". 
The lifeguard begins this short dialog by saying,
 
"Sir, please. ___( followed by a phrase from which we are to choose among the answer choices)___.
Then, "Sr. García" {"Mr. García"} responds by saying: 
"Está bien." (which, in this context, is Spanish for: "Okay." ; or, "All right." 
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The answer choices provided are:
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Choice: [A]: "Nade cerca de la costa."
 → "Swim near the coast."
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Choice: [B]: "Usa bloqueador solar." → "[It /he] uses sunblock."
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Choice: [C]: "Nada con otras personas." 
 → "[It /he] swims / [is swimming] with other people.
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Choice: [D]: "Usa gafas de sol." → "[It /he] uses sunglasses." 
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Note:
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→ The only answer choice that makes sense is:
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→ Answer choice: [A]: "Nade cerca de la costa."
 → "Swim near the coast."
→ Furthermore, this "conjugation"; that is, "Nade" ; is a "command form" ; of the Spanish verb, "nadar" — which means: "to swim" — that corresponds with the "Ud." {that is, "usted"} pronoun → which is the Spanish subject pronoun for the third person, single, [formal use of the word: "you" ; when addressing or referring to ONE person] .
Since this is "lifeguard" initiating the conversation—in a "formal tone":
→ [e.g. "Señor, por favor. ________" ; 
→ and from the [aforementioned] translations of all the other answer choices provided; and that "Answer choice: [A]" is the only choice with a "command form" conjugation; and phrase that makes sense; and correct and appropriate "command form" — and text consistent with the context of the conversation; 
→ We determine that the correct answer is: 
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Answer choice: [A]:  "Nade cerca de la costa."
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