A question about rust indicator

Rust indicator will react with Fe3+ (but not Fe2+) to form intense blue colour (Prussian blue).
therefore, answer should be B.

The Fe2+ can be easily oxidize to Fe3+ in the presence of air and water.

Information found by wikipedia (for your reference)

Potassium ferricyanide is the chemical compound with the formula K3[Fe(CN)6]. This bright red salt consists of the coordination compound [Fe(CN)6]3-.[2] It is soluble in water and its solution shows some green-yellow fluorescence.

The compound has widespread use in blueprint drawing and in photography (Cyanotype process). Iron and copper toning involve the use of potassium ferricyanide. Potassium ferricyanide is used as an oxidizing agent to remove silver from negatives and positives, a process called dot etching. In color photography, potassium ferricyanide is used to reduce the size of color dots without reducing their number, as a kind of manual color correction. The compound is also used to temper iron and steel, in electroplating, dyeing wool, as a laboratory reagent, and as a mild oxidizing agent in organic chemistry. It is also used with sodium thiosulfate (hypo) to reduce the density of negative where the mixture is known as Farmer's reducer. Variants of Farmer's reducer can also be used as the intermediate step in reversal photography to dissolve the silver image produced by the first development.

Potassium ferricyanide is also one of two compounds present in ferroxyl indicator solution (along with phenolphthalein) which turns blue (Prussian blue) in the presence of Fe3+ ions, and which can therefore be used to detect rust. It is possible to calculate the number of moles of Fe3+ ions by using a colorimeter, because the very intense color of Prussian blue Fe4[Fe(CN)6]3.

Potassium ferricyanide is often used in physiology experiments as a means of increasing solution's redox potential (Eo' ~ 436 mV at pH 7). Sodium dithionite is usually used as a reducing chemical in such experiments (Eo' ~ -420 mV at pH 7).

Potassium ferricyanide is the main component of Murakami's etchant for cemented carbides.

Prussian blue, the deep blue pigment in blue printing, is generated by the reaction of K3[Fe(CN)6] with ferrous ions