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The lead formed between the left arm and left leg is Lead 3. In standard electrocardiography, Lead 3 is an example of a bipolar limb lead, which records the electrical activity of the heart by measuring the voltage difference between the left arm and the left leg. This lead is particularly useful for analyzing the inferior part of the heart.

In Lead 3, the positive electrode is placed on the left leg, while the negative electrode is placed on the left arm. This configuration allows for efficient detection of electrical impulses that travel through the heart, providing crucial information about its function and rhythm.

To clarify the context of the other options, Lead I measures the potential difference between the left arm and the right arm, while Lead aVL is an augmented unipolar lead that measures the electrical potential at the left arm in relation to a central point between the other two limb electrodes. Lead II records the electrical activity from the right arm to the left leg. Each of these leads provides valuable information, but specifically, the lead that connects the left arm and left leg is indeed Lead 3.