The correct pair is:

The Kushans ... Gandhara School of Art

The Kushan Empire, which existed from the 1st to the 3rd century CE in the region of present-day Afghanistan, Pakistan, and Northern India, was known for its patronage of the Gandhara School of Art. The Gandhara School of Art flourished in the Gandhara region (modern-day Pakistan and Afghanistan) during the Kushan period. It was heavily influenced by Hellenistic and Roman artistic styles and produced iconic sculptures and reliefs depicting Buddhist themes.

The other pairs mentioned are not correctly matched:

- Harappan Civilization is associated with the Indus Valley Civilization, which thrived around 2600 to 1900 BCE in the region of present-day Pakistan and Western India. The Painted Grey Ware is associated with the later Vedic period (around 1000 to 600 BCE) and is not related to the Harappan Civilization.

- The Mughals were a dynasty that ruled much of the Indian subcontinent from the 16th to the 19th century. They were not associated with the Ajanta Paintings. The Ajanta Caves, located in Maharashtra, India, are renowned for their ancient Buddhist rock-cut cave paintings and sculptures that date back to the 2nd century BCE to 6th century CE.

- The Marathas were a prominent regional power in Western India during the 17th and 18th centuries. They were not associated with the Pahari School of Painting. The Pahari School of Painting refers to a style of miniature painting that developed in the hilly regions of Himachal Pradesh, Jammu and Kashmir, and Uttarakhand in Northern India.

Therefore, the correct pair is "The Kushans ... Gandhara School of Art."