What do the stereochemical labels D and − mean in tartaric acid? Why is the isomer of tartaric acid above labelled as ᴅ and (−)?

it rotates plane-polarized light clockwise
it rotates plane-polarized light anticlockwise
it is an equimolar mixture of the enantiomorphs
its configuration is related to that of ᴅ-glyceraldehyde


If I am not wrong, the ᴅ in ᴅ-glyceraldehyde refers to (+)-glyceraldehyde, and likewise ʟ-glyceraldehyde is (−)-glyceraldehyde. The (+) and (−) labels indicate that these isomers are dextrorotatory and levorotatory, respectively.
So in ᴅ-(−)-tartaric acid, what does the ᴅ mean, if not dextro, and what does the (−) mean, if not levo (the minus sign indicating anticlockwise rotation of plane-polarized light)? How can one molecule be both?
As far as I remember, small d always means the same as (+), not small cap ᴅ. So, a dextrorotatory compound is prefixed with either (+) or d while a levorotatory compound is prefixed with either (−) or l.

Small caps ᴅ and ʟ prefixes have different meaning: if an enantiomer can in principle be synthesized from naturally occurring (+)-glyceraldehyde without affecting its configuration, it is considered the ᴅ-form. Otherwise, it is considered the the ʟ-form. And, say, the ᴅ-form of a molecule is not necessarily a dextrorotatory one.

So, the answers to your question are options 2 (the (−) sign indicates that it rotates plane-polarized light anticlockwise), and option 4 (the ᴅ label indicates that it is related to ᴅ-glyceraldehyde).