Which of the following options describes a characteristic of parcels that are deemed non-machinable?

The correct choice accurately highlights that non-machinable parcels are typically oversized and unusually shaped. Non-machinable items are those that cannot be efficiently processed by the automated mail sorting systems due to their size, shape, or construction. This includes packages that are excessively large, have irregular dimensions, or consist of materials that impinge on the automated machines' sorting process.

While other options speak to characteristics often found in shipping, such as weight and material composition, they do not directly address the fundamental aspect that makes an item non-machinable. For instance, while plastic-sealed parcels might be more difficult to handle manually, they are not inherently categorized as non-machinable. Weight under a certain threshold does not determine machinability either; it's more about the overall size and shape. Similarly, the fact that a parcel is made from biodegradable materials does not influence its ability to be processed by machines. Thus, the defining feature of non-machinable parcels is indeed their oversized or unusually shaped nature, making the choice regarding size and shape the most relevant.