Avast majority of brake light and turn signal systems are equipped with:

a) xenon lighting system
b) halogen headlight bulbs
c) led lighting
d) replaceable incandescent bulbs

Replaceable incandescent bulbs

Explanation:

Incandescent bulbs have been used for over a century now. It comprises of the presence of a filament in an enclosed glass. When the filaments are being heated, light is produced.  

These types of bulbs work effectively on dimmers, timers and other control systems. When there is the need for portable lighting as headlamps and flashlights, they can be effectively used.

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because the early people found out about it.