Final answer:
In Marketo, the available custom field types include Integer, String, and Date/time, while Multi-select is NOT an available field type for creating custom fields.
Step-by-step explanation:
The field type that is NOT available when creating a custom field in Marketo is multi-select. Marketo allows the creation of custom fields to tailor the marketing automation to the unique needs of an organization. The typical field types offered by Marketo include Integer, which is for numerical values without decimals; String, which is used for text; and Date/time, which captures dates and times. However, 'Multi-select' is not a standard field type option in Marketo as it does not inherently support a field that allows multiple selections from a set of predefined options.