Select the most appropriate meaning of the underlined idiom.

Half a million dollars is only a drop in the bucket to what is required for this industry.

A very small part of something big or whole

The most appropriate meaning of the underlined idiom "drop in the bucket" in the given sentence is a very small part of something big or whole.

The other options are incorrect.

• An unreliable person is not a drop in the bucket. An unreliable person is someone who cannot be trusted.
• Unity and functioning together to achieve a goal is not a drop in the bucket. Unity and functioning together to achieve a goal is called synergy.
• A person or situation having no hope of positive change is not a drop in the bucket. A person or situation having no hope of positive change is called a lost cause.

The idiom "drop in the bucket" comes from the idea of a drop of water in a bucket. In the same way, a drop in the bucket is a very small part of something big or whole.