The questions - which confuse students the most are those that are phrased in different codes that only trained professionals understand.

Correct sentence- The questions that confuse students the most are those that are phrased in different codes that only trained professionals understand.

If the phrase "which confuse students the most" were nonrestrictive, a comma preceding "which" would be appropriate. However, because the phrase is actually restrictive, limiting "questions" to a subgroup (those questions confusing to students), the word "that" is the best choice, though no dash is required since there is no closing dash elsewhere in the sentence that would complete the offsetting of the nonrestrictive clause.

Weavers are one of the most effective workers possessing skills and are hard working.

Correct sentence- Weavers are one of the most effective workers, possessing skills and are hard-working.

The subordinate clause "possessing skills and are hard-working" must be separated from the independent clause by a comma.

Bell peppers were originally small and green, they became large and multicolored only during the twentieth century.

Correct sentence- Bell peppers were originally small and green, becoming large and multicolored only during the twentieth century.

The first sentence becomes grammatically correct if it uses a semicolon next to "green," as the two sentences are closely related independent clauses, indicating a strong connection between the two ideas. The correct sentence uses a comma and a participle phrase ("becoming large and multicolored only during the twentieth century") to provide additional information about the transformation of bell peppers. .

Brendan was not only a great dancer and he was a great choreographer.

Correct sentence- Brendan was not only a great dancer, but he was also a great choreographer.

Saying that something is not only a dancer but a choreographer means that he is a dancer as well as a choreographer.