Q.Warehousing is regarded as an auxiliary to trade mainly because it bridges the gap of:

Each auxiliary to trade removes a different obstacle to smooth trade — warehousing's specific role concerns exactly when goods are available versus when they are needed.

Warehousing exists specifically because goods are usually produced at one time (often seasonally) but consumed throughout the year, or vice versa — it is the auxiliary to trade that bridges this gap of time by holding goods safely until they are needed. This is distinct from transport, which bridges the gap of place between where goods are produced and where they are consumed. Options (c) and (d) describe obstacles addressed by other auxiliaries to trade (communication, banking/foreign exchange), not warehousing.

Confusing warehousing's role (bridging time) with transport's role (bridging place) — both are auxiliaries to trade but solve different problems.