When a switch is closed, completing an lr series circuit, the time needed for the current to reach one half its maximum value is time constants?

The answer is 0.693 .  When a switch is closed, completing an LR series circuit, the time needed for the current to reach one half its maximum value is 0.693   

e^(-t/T) = 0.5 
-t/T = ln(0.5) = -0.693 

Ocr stands for optical character recognition. so the choice is straight forward.