The correct answer is: B. have been producing/become
The sentence is about the past and the future. The manufacturers have been producing electric cars for a number of years, but they are not yet common. It is likely that they
will become common in the future.Option A
is incorrect because the simple past tense “produced” does not express the idea that the manufacturers have been producing electric cars for a number of years.Option C is incorrect because the simple present tense “produce” does not express the idea that it will be a long time before electric cars become common.
Option D is incorrect because the present progressive tense “are producing” does not express the idea that the manufacturers have been producing electric cars for a number of years.
Option E is incorrect because the past perfect tense “were producing” does not express the idea that it will be a long time before electric cars become common.