On some early automobiles both headlights failed when one bulb burned out. The headlights were likely connected in

Series.

Explanation:

In the series combination, the each component is connected through a single path and the current flow through the circuit is same but the voltage is dropped across each component and the overall voltage will be the sum of all the component's voltage.

In this type of circuit every component is connected to each other if one bulb is burn out then all the circuit will fuse.

Therefore, the headlights of few early automobiles failed when one bulb burned out because those headlights were connected in series circuit.