The correct answer is d) Lepidoptera.
Lepidoptera, which includes butterflies and moths, is well known for having siphoning mouthparts. These specialized mouthparts allow them to feed on nectar by extending a long proboscis into flowers.
While some species in the other orders listed, such as certain types of flies in Diptera and bees in Hymenoptera, may possess mouthparts that are adapted for similar feeding methods, it is primarily the Lepidoptera order that is characterized by the siphoning adaptation as a distinctive feature.