Resistant plasmodium falciparum malaria in the pediatric age group should be treated with :
Treatment of resistant falciparum malaria in children :
i) Child < 8 years of age :

Quinine + Clindamycin or
Atovaquone + Proguanil.

ii) Child  > 8 years of age

Quinine + Doxycycline / Tetracycline or
Atovaquone + Proguanil.

Note - Clindamycine can be used in place of doxycycline or tetracycline.
Coming to the question

Chloroquine is not used for resistant falciparum malaria.
Doxycycline and tetracycline are used in children only above the age of 8 years.
Clindamycin can be used in both, above and below 8 years
As age of the child has not been mentioned in the question, the single best answer is clindamycin.
Chloroquine
Tetracycline
Clindamycin
Doxycycline