Question

A wooden clapper is used to

Answer 1

Wooden clappers in sowing are used to force in the steam after pressing and to absorb moisture. 

Answer 2

The answer is B. force in the steam after pressing.
 In order to use the wooden clapper, you need to press your seam with a hot iron and produce lots of steam and it will trap the steams in the fabric.
This will make your cloth feels amazingly crisp, flat pressed seams,  unwrinkled and creases.