Steven Westminster helped pioneer the exporting of electronics from China in 1976, the year after Mao Zedong died, and ever since, his Silicon Valley consulting firm, searching for other countries, from Malaysia to Bangladesh, is capable of supplying the big electronics firms in the United States.
Meaning Analysis - SW -started the export of electronics from china in 1976. From 1976 till now , his consulting firm has been searching for countries for export options. (Import to US for supplying electronics to US firms).
A)died, and ever since, his Silicon Valley consulting firm, searching for other countries, from Malaysia to Bangladesh, is capable of supplying the big electronics firms in the United States
-- "ever since" symbolizes the continuity but we have used a present tense and verb-ing modifier here.
-- due to the sentence structure , it appears that "Consulting Firm" is capable of supplying electronics. This is not the intended meaning.
B)died; ever since, his Silicon Valley consulting firm has searched for other countries, from Malaysia to Bangladesh, that has the capability of supplying the big electronics firms in the United States
-- that is modifying "countries" and so should not be using "has".
-- wordy - "someone is capable" is better than "someone has the capability" - Not very imp point but making a note.
C)died, and ever since, his Silicon Valley consulting firm has searched for other countries, from Malaysia to Bangladesh, capable of supplying the big electronics firms in the United States
-- Correct Answer
D)died, and ever since, his Silicon Valley consulting firm has searched for other countries, from Malaysia and Bangladesh, that are capable of supplying the big electronics firms in the United States
-- firm has been searching for counties that are capable - "tense forms" - are / has been doesn't gel here. The question is not - whether the country is capable now. Countries must be capable in past when they were choose.
E)died; ever since, his Silicon Valley consulting firm has searched for other countries, from Malaysia and Bangladesh, having the capability of supplying the big electronics firms in the United States
-- "capable of supplying" is better construction than "having the capability of supplying".