The question refers to the statement, "Which cannot be done directly, cannot be done indirectly," which is a fundamental principle associated with the doctrine of "colourable legislation" in law. Let's explore the details and reasoning behind this doctrine to understand why it is the correct answer.

1. Understanding Colourable Legislation:
   • The doctrine of colourable legislation is based on the principle that what a legislature cannot do directly, it should not be able to accomplish indirectly.
   • This doctrine is often applied in the context of a federal structure, particularly in India, where the Constitution mandates a distribution of powers between the Union and State Legislatures.
   • It ensures that while enacting laws, a legislature does not exceed its competence as defined by the Constitution and that the separation of powers is respected.
2. Application of the Doctrine:
   • If a legislative body tries to pass a law that it has no competence to pass under the guise of a different subject over which it does have competence, such legislation is termed as "colourable legislation."
   • The judiciary often examines the true character of the legislation and determines whether it falls within the legislative competence of the body enacting it.
3. Example and Judicial Interpretation:
   • For instance, if a State Legislature enacts a law purportedly concerning 'taxation' which is actually intended to regulate 'interstate trade', it may be deemed colourable if the latter falls outside the state's jurisdiction.
   • Court decisions, such as the K.C. Gajapati Narayan Deo case, have upheld this doctrine to invalidate legislation crafted to disguise its true intent or exceed constitutional bounds.
4. Exclusion of Other Options:
   • Pith and Substance: This doctrine deals with determining the true nature or substance of legislation to decide its validity, not colourability.
   • Harmonious Construction: This refers to resolving conflicts between different laws or provisions harmoniously, rather than addressing legislative competence.
   • Doctrine of Eclipse: It pertains to the temporary invalidation of a law due to inconsistency with fundamental rights, which may revive if the inconsistency is removed, and is unrelated to indirect legislation.

Hence, the statement "Which cannot be done directly, cannot be done indirectly" epitomizes the doctrine of colourable legislation.