The correct active form of the given sentence is A speeding car knocked down an elderly lady this morning.

The sentence "An elderly lady was knocked down by a speeding car this morning" is in the passive voice, with the subject "an elderly lady" and the verb "was knocked down." The passive voice is formed by putting the subject of the active sentence after the verb "to be" and the past participle of the main verb. In this case, the subject of the active sentence is "a speeding car" and the main verb is "knock down." The past participle of "knock down" is "knocked down." So, the passive form of the sentence is An elderly lady was knocked down by a speeding car this morning.

The active form of the sentence puts the subject of the sentence, "a speeding car," before the verb "knock down." The verb "knock down" is followed by the direct object, "an elderly lady," and the prepositional phrase "this morning."

The other options are incorrect because:

• A speeding car knocks down an elderly lady this morning. is in the present tense, while the original sentence is in the simple past tense.
• A speeding car had knocked down an elderly lady this morning. is in the past perfect tense, while the original sentence is in the simple past tense.
• A speeding car was knocking down an elderly lady this morning. is in the past progressive tense, while the original sentence implies that the knocking down has already happened.