<p>Although measures have been introduced to discourage the use of mobile telephones inside the opera house, <strong>their</strong> effectiveness remains limited.</p>
<p>In this sentence, we need a pronoun to refer back to the "measures" that have been introduced. The pronoun should agree in number and gender with the antecedent "measures." Since "measures" is plural and does not have a specific gender, the pronoun "their" is the correct choice. </p><p>The pronoun "they" (option A) is also plural, but it is a subject pronoun and cannot be used to refer back to a noun. </p><p>The pronoun "them" (option C) is also plural, but it is an object pronoun and cannot be used to refer back to a noun. </p><p>The pronoun "theirs" (option D) is a possessive pronoun and cannot be used to refer back to a noun. Therefore, the correct answer is <strong>their</strong>.</p>