Which of the following best defines an attenuated vaccine?

A live pathogen weakened by mutations in virulence genes so it no longer causes disease but can still replicate and stimulate immunity.

A toxoid vaccine generated by detoxifying a bacterial toxin by formaldehyde treatment to render the toxin harmless while preserving antigenic structure to induce neutralizing antibodies.

A subunit vaccine composed of purified proteins or polysaccharides from a pathogen that stimulate primarily antibody production and require adjuvants to be effective.

An inactivated organism that is chemically or thermally killed so it cannot replicate and thus relies on repeated doses to maintain immunity over time.