The pre-fixes EN and EM mean "into" and "in". Before labial sounds such as b, m, and p, the prefix "en-" becomes "em-" as it is easier to pronounce "m" before them than a "n".
Option B is correct. Both words begin with "b" and thus require the prefix "em-".
Options A, C and D are incorrect. As stated before, it is difficult to pronounce the "n" sound words that begin with "b", "p" and "m". Thus, the prefix "en-" should not be used.