A monkey has been trained to perform a visual spatial selective attention task. A microelectrode is placed in its visual cortex to record the activity of single neurons. When the monkey selectively attends a stimulus flashed within the recorded neuron’s receptive field, the investigator will observe that the recorded neuron’s firing rate

The monkey's neuron's firing rate will increase because the animal is highly aroused in hopes of a reward.

Explanation:

As the exercise describes, we have a monkey who has been trained to perform visual spatial selective attention task. We can assume that, during this training, the monkey was given a reward when things went well and the monkey behaved correctly. So, when the monkey  selectively attends a stimulus flashed within the recorded neuron’s receptive field, the investigator will observe that the recorded neuron’s firing rate has fired up because the animal is aroused in hopes of a reward. Neuron's activities increase when the individual is, for example, under excitement as the monkey was.

rain should be the answer.