Which layer would be the most suitable to be responsible for 

"When a user enters a password on a mobile device the system must show individual symbols as dots."

Pick the best choice for the following statement

"The system's grade entry fields shall accept a student's grade entry with up to two decimal places."

In a clean architecture, code in more concrete layers can depend on code in more abstract layers, but not vice versa.

Pick the best choice for the following statement

"The system shall help students study."

Even if we cannot limit the classes involved in the operations of some class, say A, to just one, designing A, so its operations involve few other classes usually leads to a better design than if many classes were involved.

A software engineer can state properties of their model in Alloy or TLA+ and have Alloy or TLA+ exhaustively search for counter-examples up to a given size of model.

The carrier set of an algebra is closed under the operations of the algebra.

In an algebra operations relate two disjoint carrier sets, i.e. they take elements from one set  say A, and generate elements in a different set, say B.

An algebra consists of a carrier set and operations.