The use of comma at the beginning of “pause is essential” and another comma at the end of the “pause” is essential to make a meaningful sentence without a grammatical error.
In above given sentences, number of subjects should be always mentioned one by one by leaving “comma” between two subjects. This is ends with use of and for the final subject and prior to it, it is essential to use one comma.
Thus, the options (d), (c) and (a) are incorrect.
In above given sentences, number of subjects should be always mentioned one by one by leaving “comma” between two subjects. This is ends with use of and for the final subject and prior to it, it is essential to use one comma.
Therefore, the correct option is – b. This semester I am taking classes in Spanish, chemistry, and biology.