In the passage about mastery of words and effective speaking, the sentence "Every speaker's aim is to be heard and _________" is best completed by which word?

Introduction / Context:
This question comes from a passage on the power of words and effective oral communication. The line being tested describes the basic goal of any speaker addressing an audience. It states that every speaker wants not only to be heard but also something more. You must choose the word that logically and grammatically completes this idea in standard English.

Given Data / Assumptions:

• The target sentence is: Every speaker's aim is to be heard and ________.
• The options are understand, understandable, understood, and understandably.
• The structure to be heard and ________ calls for a parallel form after and.
• Heard is a past participle used in a passive infinitive: to be heard.

Concept / Approach:
In English, when we join two passive infinitives with and, we normally repeat the past participle form: to be heard and understood, to be seen and appreciated, and so on. The word understand is a base form verb and does not fit this pattern. Understandable is an adjective meaning easy to understand, and understandably is an adverb. Neither of those continues the passive structure that started with heard. The aim of a speaker is clearly to be heard and understood by the audience, so understood is the only grammatically and logically correct option.

Step-by-Step Solution:
Step 1: Identify the grammatical pattern: is to be heard and ________.Step 2: Notice that heard is a past participle used with be in a passive infinitive construction.Step 3: Maintain parallel structure. The phrase should read to be heard and understood, with both participles matching in form.Step 4: Test each option. Understand yields to be heard and understand, which breaks the grammatical pattern.Step 5: Understandable and understandably are an adjective and an adverb; they cannot directly follow and after be heard without changing the structure. Understood is a past participle and completes the passive infinitive correctly.

Verification / Alternative check:
Read the full passage idea with the correct choice: The most powerful and perfect expression of thought and feeling through oral language comes from mastery of words. Nothing is better suited to lead speakers and readers into control of English than the command of phrases that exactly express thought. Every speaker's aim is to be heard and understood. This version is clear and idiomatic. Substituting understandable would produce to be heard and understandable, which describes the speaker as understandable but breaks the passive verb pattern. Using understandably would suggest to be heard and understandably, which is not meaningful. Only understood preserves the natural collocation.

Why Other Options Are Wrong:
Understand is a bare infinitive and cannot simply follow and in this structure; the sentence would need a different construction, such as to be heard and to understand others, which is not what the passage says. Understandable and understandably change the part of speech and create awkward phrasing that does not match the original focus on what happens to the speaker words in the minds of listeners. The passage is clearly concerned with whether the audience grasps the meaning, that is, whether the words are understood.

Common Pitfalls:
Many learners confuse the past participle understood with the base verb understand, especially in passive constructions. A simple trick is to check if the first verb in the pair is a past participle; if so, the second usually should be too. Recognising patterns like heard and understood, seen and admired, or respected and remembered can speed up your decision in exam conditions.

Final Answer:
The sentence should read Every speaker's aim is to be heard and understood, so the correct option is understood (option C).