Garbhagriha
The Garbhagriha, also known as the sanctum sanctorum, is a small enclosed space within a temple where the main deity or idol is placed. It is considered the most sacred and important part of a Hindu temple. In this case, the answer to the question is option B - Garbhagriha.

Explanation:
The given question states that the place being referred to is where only the king, his immediate family, and priests gathered for ritual worship. Based on this information, we can deduce that it is a place of great importance and exclusivity.

1. Shikhara:
The Shikhara refers to the towering spire or peak of a Hindu temple. It is the most prominent and visible part of the temple structure. While it is an important architectural feature, it is not a place for ritual worship. Therefore, option A - Shikhara is incorrect.

2. Garbhagriha:
The Garbhagriha is the innermost and holiest part of a temple. It houses the main deity or idol of worship. Only the king, his immediate family, and priests are allowed inside this sacred space for performing rituals and offering prayers. Hence, option B - Garbhagriha is the correct answer.

3. Superstructure:
The superstructure refers to the upper portion of a temple that rises above the Garbhagriha and encloses it. It often consists of multiple tiers or levels with intricate architectural details. While it adds grandeur to the temple, it is not a specific place for ritual worship. Therefore, option C - Superstructure is incorrect.

4. Pietra Dura:
Pietra Dura is a decorative art form that involves inlaying semi-precious stones into marble or other stones to create intricate designs and patterns. It is not a place of worship but rather a technique used in artistic craftsmanship. Therefore, option D - Pietra Dura is incorrect.

In conclusion, the correct answer to the given question is option B - Garbhagriha, as it is the sacred space within a temple where only the king, his immediate family, and priests gather for ritual worship.