a.

1. 严格遵守……的[（+of）]
   A careful driver is observant of the traffic rules.

   谨慎小心的驾驶者严格遵守交通规则。

2. 观察力敏锐的
   An observant journalist notices things that other people may not be aware of.

   观察敏锐的记者会注意到其他人也许没察觉的事物。

3. 当心的，注意的
   The nurse is very observant to her patient's needs.

   那位护士非常注意她的病人的需求。

adj.

1. quick to notice things.
2. observing the rules of a religion.

n.