Which introductory phrase best completes the sentence? aprendas a ahorrar. no hay duda que estoy seguro que es verdad que es importante que

The right answer is the D) Es importante que.

Explanation: Just to elaborate a little on the answer, it can be added that this phrase, which can be translated as "It is important that," best completes the rest of the sentence "you learn (how) to save." "Estoy seguro que" can be translated as "I am certain that," and it would require the use of another verb tense, such as the future, but not the subjunctive ("aprendas" is the subjunctive of the verb "aprender," and it can be understood as a suggestion or a possibility, but not as a fact). In addition, "Es verdad que" can be translated as "It is true that," and it does not complete the rest of the sentence in a grammatically correct way. Finally, "No hay duda" would also require the use of another verb tense, such as the future, since it can be translated as "There is no doubt that."