Why did the Soviets not recognize the Operation Reinhard camps? They had been blown up.
They looked like military facilities.
They had been destroyed, fields planted and houses built.
They did not reach these areas.
American troops discovered these camps.

C. They had been destroyed, fields planted and houses built.

Explanation:

Operation Reinhard camps was a systematic codename for the secret German plan headed by Reinhard Heydrich to massacre (exterminate) Polish Jews during the World War II i.e from October, 1941 to November, 1943 in German-occupied Poland.

The introduction of an extermination camp to execute the secret German plan of mass murder was considered to be the deadliest (lethal) period or phase of the Holocaust.

Under Operation Reinhard, the extermination camps were Sobibór, Treblinka, Bełżec, Chełmno, Auschwitz II and Majdanek.

In order to cover up the massacre (mass murder) of the Polish Jews, the Nazi army developed a secret plan termed the Sonderaktion 1005, also known as Aktion 1005 (exhumation action) so as to remove all the signs and traces.

Hence, the Soviets did not recognize the Operation Reinhard camps because they had been destroyed, fields planted and houses built.