An outside micrometer is built to measure the external size of an object, gripping it between a fixed anvil and a moving spindle that are carried on a C-shaped frame.

Both measuring faces point toward each other on the open side of the frame, so the instrument is designed to close onto a surface from the outside, such as the diameter of a rod.

A deep internal groove, by contrast, is a narrow recess cut down inside the workpiece, and the bulky C-frame with its anvil and spindle simply cannot enter that confined space.

Because the spindle and anvil cannot reach into the groove, the micrometer has no way to touch the two faces whose distance is to be measured.

The tool does not expand to fit into grooves (option 1); its opening is set only by turning the thimble, and it never spreads outward to grip an internal surface.

It is not made for large holes (option 2), which need an inside micrometer or bore gauge, and it can measure only one dimension at a time, never several grooves at once (option 4).

Hence, the answer is The spindle cannot reach into grooves.