Can you share with us what you have achieved so far? Your workflow or example data would help a lot in answering your question. How do you define “most common genre”? This link about minimal reproducible workflows might help:
Reproducible (Minimal) Workflow Example - KNIME Resources / Knowledge sharing - KNIME Community Forum
I’ll do a speculative guess though:
Set the Cell Splitter to “List” or “Set” instead
Ungroup that List
GroupBy (genre as group column, aggregate any column with count)