The correct answer is option 'c) Terribly'.

Explanation:
The word 'loud' is an adjective that describes the noun 'class'. In this sentence, we are trying to convey that the class is very loud today.

To complete the sentence correctly, we need an adverb that describes the adjective 'loud'. Adverbs modify adjectives, verbs, or other adverbs, providing more information about how, when, or where something happened.

In this case, we need an adverb that describes the degree or intensity of loudness. The adverb 'terribly' is the correct choice because it means 'very' or 'extremely', indicating a high degree of loudness.

Let's analyze the other options to understand why they are incorrect:

a) Terrible: This is an adjective, not an adverb. It describes something of poor quality or causing distress. While it can be used to describe a loud noise, it doesn't convey the same degree of loudness as 'terribly'.

b) Terrific: This is an adjective with a positive connotation, meaning excellent or very good. It doesn't convey the idea of loudness.

d) So terrible: This phrase is not grammatically correct. 'So' is an intensifier that can be used with adjectives to indicate a high degree, but it needs to be followed by an adjective. In this case, 'terrible' is an adjective, not an adverb, so it doesn't fit grammatically.

In conclusion, the adverb 'terribly' is the correct choice to complete the sentence because it accurately describes the high degree of loudness in the class today.