The correct option is B Dearly
Option
B is the correct answer because ‘dearly’ means ‘expensive’. The weakness in
their defense turned out to be quite expensive. Thus, option B is the right adverb
for this sentence.
Option A is incorrect because ‘dear’ is an adjective and it is not the
appropriate answer for this sentence.
Option C is incorrect because the word ‘deary’ does not exist in English language.
Option D is incorrect because ‘dearness’ is a noun and it is not the
appropriate answer for this sentence.