The correct answer is option 'D', Indo-Islamic Style.

The Indo-Islamic architectural style is a blend of Indian and Islamic architectural elements that developed in the Indian subcontinent during the medieval period. It is characterized by the fusion of Hindu, Buddhist, and Jain architectural styles with Islamic influences brought by the Delhi Sultanate and later the Mughal Empire. This style can be further categorized into four distinct sub-styles:

1. Imperial Style (Delhi Sultanate):
- This style emerged during the reign of the Delhi Sultanate (1206-1526) and is associated with the early Islamic rulers in India.
- The architecture of this period reflects a unique blend of Indian and Islamic features.
- Examples of the Imperial Style can be seen in structures such as the Qutub Minar in Delhi and the Alai Darwaza at the Qutub Complex.

2. Provincial Style (Mandu, Gujarat, Bengal, and Jaunpur):
- The Provincial Style developed in various regions of India under different Islamic dynasties.
- It is characterized by the integration of local architectural traditions with Islamic elements.
- Notable examples of the Provincial Style include the Jahaz Mahal in Mandu, the Adalaj Stepwell in Gujarat, and the Adina Mosque in Bengal.

3. Mughal Style (Delhi, Agra, and Lahore):
- The Mughal Style reached its peak during the reign of the Mughal Empire (1526-1857).
- It is known for its grandeur, symmetry, and the extensive use of white marble and red sandstone.
- Iconic structures like the Taj Mahal, Red Fort in Delhi, and the Lahore Fort represent the Mughal Style.

4. Deccani Style (Bijapur, Golconda):
- The Deccani Style emerged in the Deccan region under the rule of various Islamic kingdoms such as Bijapur and Golconda.
- It is characterized by the use of local materials, intricate ornamentation, and a unique blend of Persian and Indian architectural styles.
- Examples of the Deccani Style include the Gol Gumbaz in Bijapur and the Charminar in Hyderabad.

In conclusion, the architectural style conventionally categorized into the Imperial Style (Delhi Sultanate), the Provincial Style (Mandu, Gujarat, Bengal, and Jaunpur), the Mughal Style (Delhi, Agra, and Lahore), and the Deccani Style (Bijapur, Golconda) is referred to as the Indo-Islamic Style. It represents the fusion of Indian and Islamic architectural elements and has left a lasting impact on the architectural heritage of the Indian subcontinent.