The answer to your question is le
your answer will be A:Mayor because it describes mi hermanito meaning little brother so there fore your mom is older which is "mayor".
¡hola!
<em>Identifica el complemento indiecto.</em>
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Los familares de los mifieros (?) enviaron <em>una nota de agradecimiento</em> a la doctora.
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a la doctora = complemento indirecto
<em>una nota de agradecimiento = complemento directo</em>
<em>-</em>
los familares de los mifieros se la enviaron.
<em>the mifieros' family send it to her.</em>
<em>-</em>
<em>the indirect object pronoun "le" change in "se" when the direct object pronoun "lo", "la", "los", "las" follows.</em>
<em>The indirect object pronoun always is before the direct object pronoun.</em>
<em>-</em>
hope this helps ☺☺☺