。
Physics is a fundamental natural science.
Literal
Physics [topic-は] foundational natural-science is.
基礎的 ('foundational') and the very similar 基本的 ('fundamental') are subtly different: 基礎 emphasizes the base/foundation that other things rest on, while 基本 emphasizes the core/essential. The ~的 suffix turns either Sino-Japanese noun into a な-adjective.