Choice B is correct. It's given that monarch butterflies can fly only with a body temperature of at least $55.0 degrees Fahrenheit left parenthesis degree upper F right parenthesis$ . Let $x$ represent the minimum increase needed in the monarch butterfly's body temperature to fly. If the monarch butterfly's body temperature is $51.3 degree upper F$ , the inequality $51.3 + x \geq 55.0$ represents this situation. Subtracting $51.3$ from both sides of this inequality yields $x greater than or equals 3.7$ . Therefore, if the monarch butterfly's body temperature is $51.3 degree upper F$ , the minimum increase needed in its body temperature, in $° F$ , so that it can fly is $3.7$ .

Choice A is incorrect. This is the minimum increase needed in body temperature if the monarch butterfly's body temperature is $53.7 degree upper F$ , not $51.3 degree upper F$ .

Choice C is incorrect. This is the minimum increase needed in body temperature if the monarch butterfly's body temperature is $50.0 degree upper F$ , not $51.3 degree upper F$ .

Choice D is incorrect. This is the minimum increase needed in body temperature if the monarch butterfly's body temperature is $48.7 degree upper F$ , not $51.3 degree upper F$ .