Prerequisites and credits are the two types of requirements that make up the LEED rating system. Prerequisites are the basic elements or strategies that must be included in any LEED certified project, regardless of the rating system or the level of certification1. Credits are the additional elements or strategies that can be included in a LEED project to achieve higher levels of performance and certification1. Credits are optional and have different point values depending on their environmental benefits and impacts2. Therefore, the correct answer is B. Prerequisites are mandatory and credits are optional.
LEED v4 | U.S. Green Building Council
LEED credit library | U.S. Green Building Council

Chlorofluorocarbons (CFCs) are a type of compound that was widely used in the 20th century in various applications, including as a refrigerant in air conditioning systems such as centrifugal chillers. CFCs are known to deplete the ozone layer when released into the atmosphere, and their use has been phased out in many countries under the Montreal Protocol. Reference: LEED Green Associate Candidate Handbook, U.S. Green Building Council resources

The location valuation factor gives extra credit for products sourced within 100 miles (160 km) of the project site, promoting local economies and reduced transport emissions.
"Products extracted, manufactured, and purchased within 100 miles of the project site are valued at 200% of their cost to encourage support for local economies."
- LEED v4 BD+C Reference Guide, MR Credit: Sourcing of Raw Materials

Graywater refers to relatively clean wastewater from domestic activities such as washing dishes, laundry, and bathing. Water from lavatory faucets falls into this category. Graywater can be treated and reused for non-potable purposes such as toilet flushing or irrigation, which can help conserve water. Reference: LEED Green Associate Candidate Handbook, U.S. Green Building Council resources

Placing grilles, grates or mats at all major entrances is an example of permanent entryway systems used to reduce dust, dirt and contaminants brought into a facility. Permanent entryway systems are designed to capture dirt and particulates from people's shoes and prevent them from entering the indoor environment.
This improves the indoor air quality and reduces the need for cleaning and maintenance. The LEED Green Associate Candidate Handbook states that one of the strategies for achieving indoor environmental quality is to "use permanent entryway systems (grilles, grates, mats) to reduce particulate matter being brought into buildings at regular entryways" [1, p. 16]. References: LEED Green Associate Candidate Handbook, [Indoor Environmental Quality | U.S. Green Building Council]