<h2>Right answer: Cuánto</h2>
This is an interrogative sentence in which an interrogative word is necessary and important to define the context.
Interrogative words are used to initiate a question and request information about someone or something (although they are also used in exclamations).
<u>The interrogative words used in spanish are listed as follows:</u>
-quién, quiénes (who) is used to ask or request information about people.
-qué (what) is used to ask about things.
-cuál, cuáles (which) is used to request information from a selection of a real or imaginary list.
-cuánto, cuántos; cuánta, cuántas(how many) they are used to ask about a quantity or quantities.
-cuándo (when) is used to ask about a moment in time.
-dónde (where) is used to ask about places.
-cómo (how) is used to ask questions like 'in what way' or 'how'
Then, according to the list above and taking into account in this sentence someone is asking information about a quantity (<em><u>amount of time</u></em> in this case), the correct interrogative word is:
<h2>Cuánto</h2>
And the complete sentence is:
<h2>
¿ <u>
Cuánto</u>
tiempo dura el contrato? </h2>
<u>How</u> long is the contract?
