An 8-year old child with normal tooth calcification and eruption has primary mandibular second molar extracted. The resulted space should be:
Need for Space Maintenance in Different Regions
First primary molar area: Space maintainers should always be placed whenever a primary molar is lost prematurely as a general rule. The potential for space loss when first primary molar is lost, depends on the different stages of eruption of first permanent molar teeth.
Second  primary  molar  area:  The  potential  for  space  loss  is  even greater when the second primary molars are lost prematurely because they normally serve as a buttress for permanent molar eruption.
Anterior  segment—primary  canine  area:  Early  loss  of  deciduous canine  is  more  common  due  to  erupting  lateral  incisors  rather  than caries. Occasionally,  roots  of  deciduous  canines  are  resorbed  and tooth  will  be  lost.  Early  loss  of  a  primary  cuspid  may  result  in  a closure  of  space  by  mesial  movement  of  the  posterior  teeth  or  by lingual  displacement  of  the  anterior  incisors  or  eruption  of  the permanent lateral into the space intended for the permanent cuspid.
An 8-year old child with normal tooth calciﬁcation and eruption has primary mandibular second molar extracted.  The resulted space should be maintained until the premolar root is 2/3 developed.
Reference: Pediatric Dentistry Principles and Practice SECOND EDITION, MS Muthu; pdf no 1001
Maintained until the premolar root is 2/3 developed
Closed slightly to accommodate the smaller premolar
Ignored, because the second premolar will erupt in a short time
Left untreated, because the difference in size between the primary molar and the premolar will compensate for any drifting that might occur