What is 'Productivity' in the context of Hockett's design features of language?

1. ক
   The ability to produce a finite set of memorized phrases
   
2. খ
   The capacity to create and understand an infinite number of novel utterances
   
3. গ
   The speed at which a child acquires a first language
   
4. ঘ
   The efficiency of communication in business settings