Explanation:
An observation is defined as the process which explains how it is happening (qualitative) and and quantitatively able to measure something about the process.
A law is defined as a verified statement that will remain true under the mentioned circumstances as it is based on the past observations that are carefully tested.
A theory is defined as the explanation of an aspect of natural world that can be tested, modified or rejected as a result of further experiments.
Hence, the given statements are classified as follows.
(a) All matter is made of tiny, indestructible particles called atoms - Theory
(b) When iron rusts in a closed container, the mass of the container and its contents does not change - Observation
(c) In chemical reactions, matter is neither created nor destroyed - Law
(d) When a match burns, heat is evolve - Observation
