A bevel protractor is a precision angular-measuring instrument consisting of a base (stock), a rotating dial with a graduated main scale, a vernier scale for fine reading, and an adjustable blade. It can measure angles typically over a wide range, but very small angles need extra help.

Why the acute angle attachment measures acute angles below 90° is correct:

• The acute angle attachment is a small removable blade/block that clips onto the instrument to allow accurate measurement of acute angles (less than 90°).
• Small acute angles are otherwise awkward to set up between the stock and blade; the attachment provides the extra reference face so the angle can be read directly and precisely on the same vernier scale.
• It effectively extends the useful measuring range down toward very small angles without loss of accuracy.

Why the other options are wrong:

• Adjusting the stock is not the purpose of this attachment; the stock is the fixed reference base of the instrument.
• Measuring linear distances is done by scales/callipers/micrometers — a bevel protractor is an angular instrument, not a length gauge.
• Locking the vernier scale is done by a separate clamp/lock nut on the protractor, not by the acute angle attachment.