King Harshavardhana's capital was at

Concept: Ancient and early-medieval Indian dynasties are identified by their administrative capital, distinct from cities where a ruler merely held religious assemblies or which had earlier served as a different dynasty's capital.

Application: Harshavardhana (r. 606-647 CE), the most celebrated ruler of the Pushyabhuti (Vardhana) dynasty who ascended the throne at Thanesar (Sthanvishvara) after his brother Rajyavardhana's death, later shifted his imperial capital to Kannauj, which became the seat of his empire's administration and remained a major north Indian political centre for centuries afterward.

Cross-check: eliminating the distractors by their own historical role confirms Kannauj as the only administrative capital among the options.

• Prayag (Allahabad) was the site of Harsha's renowned quinquennial charity assembly, a ceremonial gathering, not his seat of government.

• Ujjain served as a significant city/administrative centre under the earlier Gupta establishment, unconnected to Harsha's own capital.

• Pataliputra had been the imperial capital of the Mauryan Empire (and an earlier Gupta capital); by Harsha's time it had lost its political prominence.