Many people hang up on telemarketers, but others will listen politely to their pitches even if they are not interested in the product. telemarketers know that anyone who agrees to listen to a pitch is more likely to buy the product, to the phenomenon.
a. risky shiftb. polarizationc. norm-of-reciprocityd. foot-in-the-doore. door-in-the-face

Telemarketers know that anyone who agrees to listen to a pitch is more likely to buy the product, thanks to the foot-in-the-door phenomenon.  

Explanation:

'Foot-in-the-door phenomenon' is the phenomenon of persuading someone to agree upon a big request after them 'agreeing to a small request'. This phenomenon is based on a principle that when a person agrees upon a small request, there is development of bond between the requester and request. This development of bond will help the requester to make request agree upon his request.  In the above case, if a person agrees to listen to the polite pitches of telemarketers, there is agreement on small request and the probability to buy a product will increase because there is development of bond between them.

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