The correct answer is option 'A': Naravardhana.

The Pushyabhuti dynasty was a prominent ruling dynasty in northern India during the 6th and 7th centuries. The dynasty is known for its founder, Naravardhana, who played a significant role in the political and cultural history of the region. Let's explore the genealogy of the Pushyabhuti dynasty in detail:

1. Naravardhana:
- Naravardhana was the founder of the Pushyabhuti dynasty and the father of Harsha, the most famous ruler of the dynasty.
- He was the son of Prabhakaravardhana, the chief ruler of Thanesar in present-day Haryana.
- Naravardhana expanded the influence of the dynasty and established diplomatic relations with neighboring kingdoms.

2. Rajyavardhana:
- Rajyavardhana was the elder brother of Harsha and the son of Naravardhana.
- He succeeded his father as the ruler of Thanesar.
- Rajyavardhana was a powerful king and expanded the territories of the dynasty through military conquests.
- However, he was assassinated in a political conspiracy, which led to Harsha ascending the throne.

3. Prabhakaravardhana:
- Prabhakaravardhana was the father of Naravardhana and the grandfather of Harsha.
- He was a renowned ruler of Thanesar and played a crucial role in laying the foundation of the Pushyabhuti dynasty.

4. Adityavardhana:
- Adityavardhana is not directly mentioned in the options, and therefore, the correct answer is not 'D.'
- However, it is worth noting that Adityavardhana was the great-grandfather of Harsha and the father of Prabhakaravardhana.
- Adityavardhana's reign is significant as he expanded the territories of Thanesar and established friendly relations with the Gupta Empire.

In conclusion, the genealogy of the Pushyabhuti dynasty begins with Naravardhana, who was the son of Prabhakaravardhana and the father of Harsha. While Rajyavardhana and Adityavardhana played important roles in the dynasty's history, they are not the starting points of the genealogy as mentioned in the given options.