Step 1: Definition of microplastics. Microplastics are small plastic particles that pose significant environmental concerns due to their persistence in ecosystems. According to UNEP guidelines, microplastics are defined as plastic particles smaller than \( 5 \, \text{mm} \) in size.
Step 2: Analyze the options. \( 10 \, \text{mm} \): Incorrect, as this size exceeds the defined range for microplastics. \( 5 \, \text{mm} \): Correct, as it is the maximum size limit for microplastics. \( 10 \, \mu\text{m} \): Incorrect, as this is within the range of microplastics but does not represent the maximum size. \( 5 \, \mu\text{m} \): Incorrect, as this is much smaller than the defined maximum size.