The correct answer is option 3: A is true but R is false.

In the Gupta period, the human figure did indeed become the central focus of sculpture. This is supported by the rich artistic tradition of this period, where sculptures of deities, kings, and other prominent figures were created with great detail and sophistication. The artists during this time had a deep appreciation for the human form and its portrayal in sculpture.

However, the statement that in the Gupta period, artists lost interest in nature is incorrect. In fact, nature remained an important theme in Gupta period art and was often depicted alongside human figures. The artists of this time period displayed a keen observation and understanding of the natural world, as seen in the intricately carved lotus motifs and other plant and animal motifs in Gupta period sculptures.

Therefore, while assertion A is correct in stating that the human figure was the pivot of Gupta period sculptures, reason R is not a correct explanation as it wrongly suggests that artists lost interest in nature.