Explanation:

To determine the correct answer, let's first understand the meanings of the given options:

a) Semi-transparent: This term refers to something that allows some light to pass through, but is not completely clear or see-through. It is partially transparent.

b) Muddy: This term is unrelated to transparency. It describes something that is dirty or covered in mud.

c) Opaque: This term refers to something that does not allow light to pass through at all. It is completely solid and not transparent.

d) Dark: This term refers to the absence or deficiency of light. It does not provide any information about transparency.

Based on the definitions provided, the correct answer is option 'c' - Opaque.

Reasoning:

The question asks for the correct answer that represents the opposite of transparency. Transparency refers to the quality of allowing light to pass through and being able to see through an object or substance. Out of the given options, only "opaque" satisfies the condition of being completely solid and not allowing any light to pass through.

Summary:

In summary, the correct answer is option 'c'. Opaque is the opposite of transparent as it does not allow any light to pass through.