When an object moves at a very high speed relative to an observer, its measured length in the direction of motion is

a. expanded

b. lengthened

c. unchanged

d. contracted

When an object moves relative to an observer, even if the object is

a worm or a turtle, its length in the direction of motion gets shorter ...

it 'contracts'.

The faster it moves relative to the observer, the shorter it gets in the

direction of motion.

It has to be moving pretty fast before the shortening becomes measurable.