Default Attributes Plugin Settings
WooCommerce plugin that sets default attribute for variable products automatically if only 1 option is in-stock.
Variable product of T-shirt with color attribute (White/Blue/Black) and size attribute (S/M/XL):
If only Black T-shirts are in-stock: default color (Black), default size (none).
If only Small T-shirts are in-stock: default color (none), default size (Small).
If only White and XL T-shirts are in-stock: default color (White), default size (XL).