Gregor Mendel's original hypotheses about the inheritance of traits were most closely supported by the particulate hypothesis. This hypothesis proposed that traits are inherited as discrete units (now known as genes) rather than being blended together. Mendel's experiments with pea plants demonstrated that traits can segregate and be passed on independently, providing evidence for this particulate nature of inheritance. The blending hypothesis, on the other hand, suggested that traits would blend together, which Mendel's data did not support.
The other terms you mentioned (chromosome hypothesis and true breeding hypothesis) are related but not Mendel's primary hypotheses. The chromosome hypothesis came later with the understanding of genetic inheritance along chromosomes, and true breeding refers to organisms that produce offspring with the same traits when self-crossed, which Mendel did use in his experiments.