Answer:
1. Axiom
Appropriate definition: An elementary mathematical truth that is assumed self-evident and not in need of proof (a).
2. Corollary
Appropriate definition: A theorem that is an immediate consequence (and perhaps special case) of another theorem (b).
3. Contradiction
Appropriate definition: A proposition that is always false (c).
4. Tautology
Appropriate definition: A proposition that is always true (d).
5. Theorem
Appropriate definition: An important true mathematical statements (e).
6. Lemma
Appropriate definition: A theorem that is needed as a stepping stone to prove a greater theorem (f).