To produce a meaningful result set without any cartesian products, the minimum number of conditions that should appear in the WHERE clause of a four-table join is 3.
Let's go through each option to understand why it is correct or incorrect:
Option A) 8 - This option is incorrect because it suggests a much higher number of conditions than necessary.
Option B) 4 - This option is incorrect because it suggests a higher number of conditions than necessary.
Option C) 2 - This option is incorrect because it suggests a lower number of conditions than necessary.
Option D) 3 - This option is correct. To avoid cartesian products and ensure a meaningful result set, at least one condition is required for each pair of tables being joined in a four-table join. Since there are three possible pairs of tables in a four-table join, a minimum of three conditions is needed.
Option E) 5 - This option is incorrect because it suggests a higher number of conditions than necessary.
The correct answer is D. This option is correct because it represents the minimum number of conditions required to avoid cartesian products and produce a meaningful result set in a four-table join.