Answer:
The correct answer is option d. "x-rays".
Explanation:
The devastating gravitational force that a black hole has against matter produces that matter rises its temperature to million degrees. As a consequence of the extreme heat that it experiences, matter glows x-rays before being engulfed by a black hole. Since no light can escape black holes, scientists can spot their presence by studying its surrounding environments including the presence of immense gravitational forces and x-rays emissions.