Common features of chordata are

The phylum Chordata is defined by a set of distinct, fundamental anatomical features that are present at some stage during their life cycle. Let us analyze each of the given options:

1. Presence of a notochord: The notochord is a flexible, rod-like structure that runs along the dorsal side of the body. It provides skeletal support and is a defining characteristic present in all chordates during at least some stage of their development (often replaced by the vertebral column in vertebrates). Thus, this feature is common to all chordates.
2. Paired pharyngeal gill slits: These are openings in the pharynx (throat region) that connect the inside of the throat to the outside environment. In aquatic chordates, they are used for filter-feeding or respiration (gills). In terrestrial chordates, they appear during embryonic development and are later modified into other structures (such as parts of the ear and neck). This is another fundamental diagnostic feature of Chordata.
3. Dorsal hollow neural system: Chordates possess a single, hollow nerve cord located dorsally (along the back) relative to the alimentary canal. In vertebrates, this develops into the central nervous system, consisting of the brain and spinal cord. This distinguishes chordates from non-chordates, which typically have a ventral, solid, and double nerve cord.

Since all three features—presence of notochord, paired pharyngeal gill slits, and a dorsal hollow neural system—are primary diagnostic characteristics of the phylum Chordata, they are all common features.

Therefore, the correct choice is all of the above.