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A
Safety chains are decorative and not legally required
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B
Safety chains are legally required as a backup in case the primary connection (pintle hook) fails — they prevent the towed unit from completely separating and becoming a free-rolling hazard on the highway
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C
Safety chains are only required on hazmat loads
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D
Safety chains replace the need for air brake connections
Why this is the answer
SAFETY CHAINS (or safety cables) are required by 49 CFR 393.70 at every trailer-to-trailer and dolly connection in a multi-trailer combination. LEGAL REQUIREMENT: Federal safety regulations mandate properly attached and adequately rated safety chains at the tongue or drawbar connection of every towed unit; FUNCTION: Primary purpose is backup retention if the pintle hook fails; if the hook opens or the ring breaks, the safety chains catch the drawbar before the units fully separate; the chains also limit how far the units can separate, keeping the disconnected trailer from swinging completely free; PROPER INSTALLATION: Chains must be CROSSED (forming an X) under the drawbar between the towing and towed units; crossing causes the chains to form a cradle that catches and supports the drawbar if the hook fails; chains must not drag on the road (a dragging chain can hook road features) but must have enough slack to allow turns without binding; RATED STRENGTH: Chains must be rated for the weight of the towed unit — a 20,000 lb trailer requires correspondingly rated chains; INSPECTION: During pre-trip, check each chain for: secure attachment at both ends; adequate but not excessive slack; freedom from kinks, cracks, or excessive wear; proper crossing configuration; correct rating for the connected load.
Source: FMCSA CDL Manual, Chapter 7, Doubles and Triples, Safety Chains