Select the most appropriate one-word substitution for the given group of words. 

Always or often happening which is unlikely to change.

Constant

Here are the explanations of the four options:

1. Repetition: "Repetition" refers to the act of repeating something, often with the intention of emphasizing or reinforcing it. While it implies a recurring occurrence, it does not necessarily convey the notion of being unlikely to change.

2. Inveterate: "Inveterate" describes a habit or behavior that is firmly established and difficult to change. It implies a long-standing or ingrained characteristic, but it does not specifically convey the idea of always or often happening.

3. Abdicate: "Abdicate" means to renounce or relinquish a position of power or responsibility. It does not relate to the concept of something always or often happening and being unlikely to change.

4. Constant: "Constant" accurately describes something that is always or often happening and is unlikely to change. It signifies a continuous or uninterrupted state or occurrence, suggesting a consistent pattern or behavior.