The correct answer is option 'C' - the inner carbon is sp2 hybridized. The reason why ketenes absorb in the infrared (IR) at a very high frequency (2150 cm-1) is explained below:

- Ketenes are compounds that contain a carbonyl group (C=O) and a vinyl group (C=C). The carbonyl group is composed of a carbon atom (inner carbon) double bonded to an oxygen atom, while the vinyl group consists of a carbon-carbon double bond adjacent to the carbonyl group.

- In ketenes, the inner carbon atom of the carbonyl group is sp2 hybridized. This means that the carbon atom has three sigma bonds and one pi bond, resulting in a trigonal planar geometry around the carbon atom. The sp2 hybridization allows the carbon atom to form three sigma bonds, one with the adjacent oxygen atom and two with the other carbon atoms in the molecule.

- The presence of the sp2 hybridized carbon atom in the ketene molecule leads to a higher frequency of absorption in the IR region. This is because the vibrational modes of the carbonyl group, which are responsible for the IR absorption, are influenced by the hybridization state of the carbon atom.

- In general, the stretching vibration of a carbonyl group is stronger and occurs at a higher frequency when the carbon atom is sp2 hybridized compared to when it is sp3 hybridized. This is due to the greater electron density and stronger bond in the sp2 hybridized carbon-oxygen bond.

- The high frequency absorption at 2150 cm-1 corresponds to the stretching vibration of the carbon-oxygen double bond in the ketene molecule. This absorption is characteristic of the presence of a carbonyl group in the compound.

In summary, the absorption of ketenes in the IR at a high frequency of 2150 cm-1 is due to the sp2 hybridization of the inner carbon atom in the carbonyl group. This hybridization leads to a stronger and higher frequency stretching vibration of the carbon-oxygen double bond, resulting in the observed absorption.