Final answer:
The proper tool used by a professional installer to install stretch in carpet is the carpet stretcher.
Step-by-step explanation:
The proper tool used by a professional installer to install stretch in carpet is the carpet stretcher. A carpet stretcher is a long-handled tool that is used to stretch carpet tightly over the floor before securing it in place. It helps to remove any wrinkles or unevenness.
The other options mentioned are also tools used in carpet installation, but they serve different purposes. A tack strip is used to secure the edges of the carpet to the floor, while a knee kicker is a tool used to push the carpet into position along the edges or in corners.
A seam iron is used to join two pieces of carpet together by melting adhesive tape placed beneath the carpet's seams. It is not used for stretching the carpet.