Question

significant figures are digits read directly from measuring instrument plus one more digit, which is ______ by the observer

Answer 1

Estimated by the observer.

Take a buret, you can read to the 0.1 mL directly from the device, but the amount may lie between the 0.1 marks so you can estimate how far between the marks the fluid lies.

Answer 2

Significant figures are digits are read directly from the measuring instrument plus  one more digit which is Estimated by the observer.