What is the volt input for ac power adapter if a unit takes 6 D Cell batteries?

A D-cell delivers around 1.5 Volt per piece. NORMALLY (but not always!) six D-cell are joined in series to make 6 x 1.5 = 9 Volts.

Only in cases where a high current is needed there may be a facility to have 2x 3 serie cells in two branches parallel thus making it 4.5 Volts.

It is quite safe to assume you will need a 9 Volt AC-powered adapter for this equipment, exceptions observed.

With kind regards,

Henk