Choose the word that is most nearly opposite in meaning to the underlined word "compulsory" and that would correctly fill the gap.

The question asks to choose the word that is most nearly opposite in meaning to the underlined word "compulsory" and that would correctly fill the gap if the original word were replaced.

The underlined word is compulsory.
Compulsory means required or obligatory.

Let's examine the given options:

• A. unimportant: not significant or essential. This is not the opposite of required.
• B. optional: available to be chosen but not obligatory. This is the direct opposite of compulsory.
• C. easy: not difficult. This is unrelated to the meaning of compulsory.
• D. fundamental: forming a necessary base or core. This is similar to compulsory, not its opposite.
• E. negligible: so small or unimportant as to be not worth considering. This is not the opposite of required.

The word that is most nearly opposite in meaning to "compulsory" is optional. If we replace "compulsory" with "optional" in the sentence, it would read: "The students were advised to register for all the optional subjects before considering any ones." This makes grammatical sense and conveys the opposite meaning.

The final answer is $\boxed{\text{B}}$