The correct answer is B. sensed.
To sense something is to perceive it through the senses, especially by feeling or hearing. In this case, the speaker is saying that they became alert because they perceived trouble around them. This could be because they heard something suspicious, saw something out of the ordinary, or felt a sense of danger.
The other options are not as accurate. To comprehend something is to understand it fully. To transcend something is to go beyond it or rise above it. To see something is to perceive it with the eyes.
In this case, the speaker is not saying that they understood the trouble around them, or that they went beyond it, or that they saw it with their eyes. They are simply saying that they perceived it through their senses.