Final answer:
The sentence employs an Antonym context clue to suggest the meaning of 'rudimentary' by contrasting it with 'advanced', indicating that the skills taught were basic.
Step-by-step explanation:
The context clue used in the sentence 'Although the algebra class was supposed to be for advanced students, the skills that were being taught were very rudimentary.' is an Antonym context clue (B). This type of clue provides the opposite meaning of the highlighted word to aid in its comprehension. The word 'advanced' serves as an antonym to 'rudimentary', thereby suggesting that the skills were basic or elementary in nature, which is opposite to what is expected of an advanced class.