In a food chain of Grass - Deer - Leopard, if 1000 Joules of energy...

According to the $10\%$ law of energy transfer proposed by Lindeman, only $10\%$ of the energy available at one trophic level is transferred to the next trophic level.

The given food chain is: Grass → Deer → Leopard.

Energy available at the producer level (Grass) $= 1000$ Joules.

Energy transferred to the primary consumer (Deer) $= 10\%$ of $1000$ Joules $= \dfrac{10}{100} \times 1000 = 100$ Joules.

Energy transferred to the secondary consumer (Leopard) $= 10\%$ of $100$ Joules $= \dfrac{10}{100} \times 100 = 10$ Joules.

Thus, the energy received by the leopard is $10$ Joules.

Answer: 10 Joules