During which century did the Basohli style transform, leading to the Guler–Kangra phase?

18th century

Answer-  18th century

The first quarter of the eighteenth century saw a complete transformation in the Basohli style, initiating the Guler–Kangra phase. This phase first appeared in Guler, a high-ranking branch of the Kangra royal family, under the patronage of Raja Govardhan Chand (1744–1773). Guler artist Pandit Seu with his sons Manak and Nainsukh are attributed with changing the course of painting around 1730–40 to a new style, usually, referred to as the pre–Kangra or Guler–Kangra kalam. This style is more refined, subdued and elegant compared to the bold vitality of the Basohli style. Though initiated by Manak, also called Manaku, his brother Nainsukh, who became the court painter of Raja Balwant Singh of Jasrota, is responsible for shaping the Guler School emphatically. The most matured version of this style entered Kangra during the 1780s, thus, developing into the Kangra School while the offshoots of Basohli continued in Chamba and Kullu, India