A mixture of tea leaves and iron fillings can be separated by using threshing. Let us take a look at other options first.
Sieving is the process of separating materials based on the particle size. For example, we can sieve out larger particles (Say sugar granules) from smaller particles (say sand), by using an appropriately sized sieve.
Hand picking is used for the removal of material that can be easily picked by hand. Say, removal of pebbles and stones from sand, etc.
Filtration is commonly used to remove solids from liquids. We can use a filter to filter out tea leaves from tea.
Tea leaves and iron fillings do not fit into any of these options. Threshing is the process by which chaff is removed from grain and we achieve a form of density separation. Here, tea leaves are lighter as compared to iron fillings and can be separated by density difference methods.
Note that it is far easier to use a magnet to remove iron fillings from the mixture.
Hope this helps.