We look forward to hear from you.

The correct answer is D. No improvement.

The sentence "We look forward to hear from you" is grammatically incorrect. The verb "to hear" takes a direct object, so the correct sentence should be "We look forward to hearing from you."

The other options are also incorrect. Option A, "We look forward to hearing," is incorrect because it does not have a direct object. Option B, "We look forward to have heard," is incorrect because it uses the past participle "heard" as a verb, which is incorrect. Option C, "We look forward to listen," is incorrect because it uses the verb "to listen," which is not the correct verb to use in this context.

The correct sentence is "We look forward to hearing from you."