The essential medicines concept is founded on the principle of ensuring that the most important medicines needed for a health system are available, affordable, and accessible to the population at all times. These medicines are selected based on their public health relevance, evidence on efficacy and safety, and comparative cost-effectiveness.

Among the options provided:

- Option 1: Ensuring availability in all dosage forms at all times best reflects this principle, as the essential medicines list aims to guarantee that key medicines are consistently available to meet the needs of different patient groups, which may require different dosage forms (e.g., tablets, syrups, injections).

- Option 2: Maximizing profit margins for pharmaceutical companies is not aligned with the essential medicines concept, which focuses on public health impact rather than commercial interests.

- Option 3: Restricting use of medicines to tertiary care centers only contradicts the core goal of improving access at all levels of healthcare, including primary and secondary care.

- Option 4: Allowing unrestricted marketing to consumers is not a principle of essential medicines and may lead to misuse and irrational use of medications.

In summary, the fundamental principle of the essential medicines concept is to ensure continuous availability of key medicines—including various dosage forms—to meet the health needs of the community efficiently and equitably.