Which word means very attentive to detail?

The word "fastidious" accurately captures the meaning of being very attentive to detail. This term is often used to describe someone who is meticulous, painstaking, and precise in their work or observations. A fastidious person typically pays great attention to the finer points and strives for excellence in their tasks, ensuring that nothing is overlooked. This quality is essential in fields where precision and accuracy are paramount.

In contrast, the other words in the list do not convey this level of attention to detail. "Casual" suggests a relaxed, unconcerned attitude, which does not include meticulousness. "Negligent" refers to a lack of attention or care, often resulting in mistakes or oversights. Similarly, "indifferent" implies a lack of interest or concern, further distancing it from the idea of being attentive and detail-oriented. Therefore, fastidious is the most fitting word for someone who is exceptionally careful and thorough in their attention to detail.