The US and Australia have jointly test fired a hypersonic missile capable of moving at a speed ...........

English ( Master) Sentence Improvement

The US and Australia have jointly test-fired a hypersonic missile capable of moving at a speed eight times more fast than sound, as part of $54-million research project.

at a speed eight times as fast as

at a speed eight times faster

with the speed eight times as faster as

at a speed eight times much fast

No improvement required

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Explanation:

Correct Answer is (b).The presence of the preposition 'than' right after the bold part confirm that an adjective of comperative degree must be used here. Option A and D get eliminated straightaway.
Option C is ungrammtical as an adjective of comperative degree is not used with 'as ... as' phrase.
Clearly, option B is the correct answer.