Solution:Speech Act theory, proposed by J.L. Austin and later developed by John Searle, is a study of how language can be used to accomplish things in the world. Option (b) explains this: people use language to do things, not just express thoughts or emotions. For example, by saying "I promise", a person is doing the act of promising. This actionoriented use of language is represented by performative verbs (c), which convey actions that are performed by the very act of uttering the verb. Hence, (b) and (c) are correct. Illocutionary force (d), on the other hand, refers to the social or psychological action performed when uttering something, such as advising, warning, or promising. Therefore, (d) is not correct.