(A). Dilute nitric acid oxidises the aldehyde and the primary alcohol of glucose to give saccharic acid (a dicarboxylic acid). Gluconic acid, which already has the C1 oxidised to COOH, is further oxidised at C6 to give the same saccharic acid. So both glucose and gluconic acid eventually give saccharic acid; the claim that glucose is not oxidised is wrong. Incorrect.

(B). Fehling's reagent is mildly basic (contains NaOH/Rochelle salt). In base, fructose undergoes Lobry de Bruyn-van Ekenstein tautomerisation via the common enediol intermediate to give D-glucose and D-mannose, both aldohexoses. These then reduce Fehling's. Correct.

(C). Acid hydrolysis of sucrose (the corresponding disaccharide) gives an equimolar mixture of D-glucose and D-fructose, called invert sugar (because the optical rotation inverts sign during the hydrolysis). Correct.

(D). Using the D-isomers, $[\alpha {]}_{\text{D-glucose}}=+52.5^{\circ }$ and $[\alpha {]}_{\text{D-fructose}}=-92.5^{\circ }$. The mean for an equimolar mixture is $\frac{52.5-92.5}{2}=-20^{\circ }$, not $-40^{\circ }$. Incorrect.

$\therefore$ Correct options are (B) and (C).