To maintain the LEED Green Associate credential, you must earn 30 continuing education hours every two years, with at least six of those hours being LEED specific. This requirement ensures that LEED Green Associates stay up-to-date with the latest developments in green building practices and LEED rating systems. Reference: LEED Green Associate Candidate Handbook, U.S. Green Building Council resources

Explanation
Renewable Energy Certificates (RECs) are tradable, non-tangible energy commodities that represent the environmental attributes of one megawatt-hour of electricity generated from renewable sources, such as wind, solar, hydro, or biomass1. RECs can be purchased by anyone, regardless of their location or utility provider, to support the development of renewable energy and reduce their carbon footprint2. RECs can also help projects earn points for the LEED v4 Innovation credit for Green Power and Carbon Offsets, which requires the purchase of at least 50% of the total building energy use from renewable sources or offsets for a minimum of five years3.
References:
Using Renewable Energy for LEED Certification
Renewable Energy Certificates For Green Building Explained
LEED v4, Renewable Energy, and Carbon Offsets

A salvaged wood door from another site qualifies under the Materials and Resources sustainable criteria of materials reuse. Materials reuse is the practice of using existing materials or products for new purposes without altering their form or composition. Materials reuse reduces the demand for virgin materials, saves energy and resources, and prevents waste generation. The other options are not applicable to a salvaged wood door from another site. Certified wood is wood that has been harvested from forests that are managed in an environmentally responsible, socially beneficial, and economically viable manner according to the standards of an accredited certification system such as the Forest Stewardship Council (FSC). Bio-based materials are materials that are derived from plants or animals, such as bamboo, cork, wool, or cotton. Waste diversion is the practice of diverting waste from landfills or incinerators by recycling, reusing, salvaging, or donating materials. References: LEED Green Associate Candidate Handbook, page 28; USGBC, [Materials and Resources], page 4.