The answer is A: Samuel Morse's telegraph...<em>was successful.</em>
A predicate adjective <em>modifies</em> the subject of the sentence and comes after the noun they are modifying. The predicate adjective in a sentence is connected to the subject by a <em>linking verb</em>. In the example of the sentence A, <em>sucessful</em> is the predicate adjective, <em>was</em> is the linking verb and <em>all that came before the verb is the </em>subject<em>.</em>