The correct answer is D. New bridges are being built by the government.
The other options are incorrect because they do not accurately reflect the current state of the bridges. Option A, New bridges were built by the government, is incorrect because it implies that the bridges have already been built, when in fact they are still being built. Option B, New bridges are building by the government, is incorrect because it implies that the bridges are being built by themselves, when in fact they are being built by the government. Option C, New bridges have been built by the government, is incorrect because it implies that the bridges have already been built, when in fact they are still being built.
The sentence “The Government is building new bridges” is in the present progressive tense, which indicates that the action is happening now. This is consistent with the fact that the bridges are still being built.