One word substitution – that which cannot be heard.

Introduction / Context:
One word substitution items require you to replace a descriptive phrase with a precise single word. Here the phrase that which cannot be heard applies to sounds or signals whose volume or frequency is beyond human hearing. The question tests your knowledge of common English prefixes and how they change the meanings of words.

Given Data / Assumptions:

The base concept is hearing, so the core root is related to audio or sound.

The phrase cannot be heard implies a negative form built from this root.

Some options are deliberately unrelated to hearing and focus instead on eating or digestion.

You must pick the word that combines the correct root with a suitable negative prefix.

Concept / Approach:
The adjective audible means able to be heard. In English, the prefix in often negates adjectives of Latin origin, turning possible into impossible or accurate into inaccurate. Thus, inaudible means not audible or cannot be heard. Edible and digestible refer instead to eating, not to hearing, and disedible is not a standard English word. Recognising the pattern audible and inaudible allows you to choose the correct one word substitution quickly.

Step-by-Step Solution:
Step 1: Identify that the key idea is about hearing, not eating or digesting. Step 2: Recall that audible means able to be heard. This matches the positive version of the phrase can be heard. Step 3: Understand that the question asks for the negative form, something that cannot be heard. Step 4: Recognise that adding the prefix in to audible forms inaudible, which directly means not audible. Step 5: Check that inaudible is a standard dictionary word used to describe extremely soft sounds or ultrasonic frequencies.

Verification / Alternative check:
Consider sample sentences. The teacher's voice was barely audible at the back of the hall shows the positive form. If the sound cannot be heard at all, we say The sound was inaudible. In technical contexts, frequencies above the human hearing range are described as ultrasonic and therefore inaudible. There is no standard word disedible in English, and edible and digestible clearly refer to food. This checking confirms that inaudible is the only option that satisfies the phrase that which cannot be heard.

Why Other Options Are Wrong:
Option A Audible means can be heard, which is the opposite of the required meaning.
Option B Edible means safe or suitable to be eaten and has no connection with sound.
Option C Disedible is not a recognised standard English word and also tries to use the wrong root.
Option E Digestible refers to something that can be digested by the stomach and again is about food, not hearing.

Common Pitfalls:
Learners sometimes rely on the negative sounding prefix dis for many words, but English uses different negative prefixes depending on the origin of the word, such as in, un, im, and dis. For adjectives like audible and visible, the standard negative forms are inaudible and invisible, not disaudible or disvisible. Studying common root and prefix pairs will save time and improve accuracy in vocabulary questions of this kind.

Final Answer:
The correct one word substitution for that which cannot be heard is inaudible.