Well, since I can't remember the answer from the Bar Exam, let's use the process of elimination.
"Slight care" is not a legal standard under any circumstances, so that's not the answer. Cancel out "slight care."
The first question to ask to resolve this question is what "duty of care," if any, did the friend from whom the bike was stolen owe to the friend who was the owner of the bike. Common law assigns a "duty of care" based on the relationship between the people, e.g. as between a hotel owner and a hotel guest, or a storage company and the person storing his goods there.
"duty of care n.: a requirement that a person act toward others and the public with watchfulness, attention, caution and prudence that a reasonable person in the circumstances would. If a person's actions do not meet this standard of care, then the acts are considered negligent, and any damages resulting may be claimed in a lawsuit for negligence.
Here we have a bailor/bailee relationship between the boy who asked someone to hold the bike (the "bailor") and the friend who held the bike (the "bailee").
"When a bailor receives the sole benefit from the bailment, the bailee has a lesser duty to care for the property and is financially responsible only if he or she has been grossly negligent or has acted in bad faith in taking care of the property. FreeDictionary
The fact that you're not PAYING your friend to watch your bike, and your friend did not ask to borrow your bike would both be reasons for a court not to require him to exercise that much care.