Final answer:
The simple sentence among the provided options is 'A) "Learn with an example."' because it contains only a single main clause that expresses one idea and can stand alone as a sentence.
Step-by-step explanation:
To determine whether the sentences are simple, compound, or complex, one must understand the definitions of each type. A simple sentence has one idea expressed in a single main clause with a subject and a predicate and can stand alone as a sentence. For instance, 'The coronavirus spread around the world in 2020' is a simple sentence even though it describes a complex event.
Looking at the options provided:
A) "Learn with an example." - This is a short simple sentence because it contains a single main clause and can stand on its own, indicating it's the correct answer.
B) "Open and honest communication is the foundation of a healthy relationship." - While this is longer, it's also a simple sentence. However, it is not one of the provided options for a simple sentence.
C) "Our cat has been scratching himself more than usual, so I checked him for fleas." - This example is a compound sentence, as it consists of two independent clauses joined by the coordinating conjunction 'so'.
D) Submit. - This does not seem to be relevant to the grammatical classification task.
Therefore, the simple sentence is A) "Learn with an example."