Though they did not speak to each other much, there was a . . . . . . . understanding between them.

The other options are incorrect for the following reasons:

• Placid means calm and peaceful. This does not accurately describe the relationship between the two people in the example.
• Contractual means based on a contract. This does not seem to be the case in the example, as the two people do not seem to have made any explicit agreements.
• Verbal means spoken. This does not accurately describe the understanding between the two people in the example, as it is unspoken.