The implication of the masculine singular form is that Hashem responded to Yitzchak’s prayer – rather than to Rivkah’s. Chazal (Yevamos 64a) teach that one cannot compare the prayer offered by a tzaddik ben tzaddik, righteous person, the son of a righteous person, (Yitzchak Avinu) to the prayer entreated by a tzaddik ben rasha (Rivkah Imeinu). Thus, Hashem listened to Yitzchak. This concept is most efficacious when each individual supplicant is praying for himself personally. Yitzchak and Rivkah were both praying for the same thing: offspring. Why would Yitzchak’s prayer find greater acceptance than Rivkah’s?
Horav Meir Shapiro, zl, explains that Yitzchak, having been raised in the home of Avraham Avinu and Sarah Imeinu, lived in a spiritual environment in which he was not exposed to evil of any sort. His parents were paragons of virtue, his home a sanctuary. Thus, when he prayed for children, he did not add “good” children, because he did not know the meaning of bad. Rivkah, on the other hand, was acutely aware of bad and evil, having grown up with a father like Besuel and a brother like Lavan. When she prayed, she stipulated “good” children. This is why Hashem listened to Yitzchak’s entreaty over hers. Since Yitzchak did not specify the kind of child he wanted, Hashem gave him two sons – Yaakov and Eisav. This is what is meant by the idea that Hashem responded to Yitzchak. Had He listened to Rivkah’s prayer, Eisav would not have existed. Since unfortunately we require Eisav and his descendants to keep us in line, Hashem listened to the tzaddik ben tzaddik.