Test Isolation Principle:
Apex test methods run in a sandboxed environment without access to existing org data unlessseeAllData=trueis used.
The test fails if it relies on data not created within the test itself.
Why Not Other Options?
A:runAssimulates user context but does not affect test isolation.
B: @future methods do not cause failures if handled correctly.
D: Syntax errors would prevent code compilation.
References:Test Data Isolation:https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode
/apex_testing_data.htm