Mark had lived in a small city for all of his life, and local people thought of him as a solid citizen. in this sentence, the word solid most likely means a. dependable. b. popular. c. unbroken. d. immovable.

It most likely means dependable.

If you replace the word "solid" in the sentence with popular, unbroken, or immovable it doesn't makes sense like the word dependable does:

Mark had lived in a small city for all of his life, and local people thought of him as a popular citizen.

Mark had lived in a small city for all of his life, and local people thought of him as a unbroken citizen.

Mark had lived in a small city for all of his life, and local people thought of him as a immovable citizen.

Those don't make sense like:

Mark had lived in a small city for all of his life, and local people thought of him as a dependable citizen.

Hope this helped