D-Glucose is an open-chain aldohexose with four stereocentres. How many of its diastereomers belong to the L-series?

There are $2^4=16$ aldohexose stereoisomers, 8 in the D-series and 8 in the L-series. Excluding D-glucose itself gives 15 other stereoisomers, but L-glucose is its enantiomer and not a diastereomer, so 14 diastereomers remain. Of these, 7 are D-sugars and $8-1=7$ are L-sugars, so the answer is 7.