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

The correct answer is C. tacit.

A tacit understanding is an unspoken agreement between two or more people. It is often based on shared values, beliefs, or experiences. Tacit understandings can be very strong, even if they are not explicitly stated.

In the given example, the two people do not speak to each other much, but they still have a tacit understanding. This understanding may be based on their shared history, their common goals, or their mutual respect.

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.