Final answer:
The niche in a mosque designating the qibla is a mihrab, an ornately decorated feature that indicates the direction of Mecca for prayer. The correct option is B.
Step-by-step explanation:
The niche or decorative panel in a mosque designating the qibla is known as a mihrab. This important architectural feature serves as a semicircular niche in the wall of a mosque and indicates the direction of Mecca, which all Muslims face during prayer. The qibla wall where the mihrab is located is typically the most ornately decorated area of a mosque.
The mihrab is not only a functional element, guiding the faithful in their prayers towards Mecca, but also often a focal point of artistic expression, featuring intricate designs and decorations, such as calligraphy, mosaics, and rich materials like marble. The correct option is B.