Infer a source from a reversed branch current

A source with emf E is in series with 3 Ω between a bottom rail at 0 V and an upper rail at 9 V. Its positive terminal faces the upper rail. The measured current is 0.50 A downward from the upper rail through this branch.

What is the source emf?

Organize the givens & assumptions

Rail voltage: 9 V
Series resistance: 3 Ω
Actual branch current: 0.50 A downward

Settled lumped DC circuit, ideal connecting wires, positive constant Ohmic resistances and no changing external magnetic flux. Sources are ideal except for explicitly specified series resistances. The source can absorb energy in its ideal model.

A $7.5\ \mathrm V$
B $10.5\ \mathrm V$
C $9\ \mathrm V$
D $1.5\ \mathrm V$

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