Option E is the correct answer because the given combination of the option is perfect because they are both in the simple past tense, which makes complete sense. "understood" is the past tense of "understand" which means to get or intend the meaning of, and "were speaking" is correct because "were" is used in the plural form ,as the past tense of "are".
Option A is incorrect because "speaks" is the third person singular form of speak, and so it is incorrect because it is used in singular form, while "they" are speaking in the statement.
Option B is incorrect for same reasons as A.
Option C is incorrect because will is used in future tense while "spoke" is used in past tense, hence it is incorrect because the statement must be in the same tense or form.
Option D is incorrect because 'are speaking" is in present tense, while "understood" is in past tense.