Answer: Scale up the virtual warehouse.

The question specifically asks about improving performance for a single large complex query. According to Snowflake documentation and the community discussion consensus (with A receiving 67% of votes and detailed reasoning), scaling up (increasing the warehouse size) is the correct approach for improving performance of individual complex queries. Scaling out (option B) and multi-cluster warehouses (options C and D) are designed to handle concurrency issues when multiple queries are running simultaneously, not for optimizing a single query. The community discussion clearly explains that 'scaling up is for improving speed of a single complex query' while 'scaling out is for solving concurrency issues'.

Author: LeetQuiz Editorial Team