The correct answer is option 1: Potassium. When a plant has yellow leaves with dead spots, it is often an indication of potassium deficiency. Potassium is an essential nutrient for plants as it helps in various physiological functions such as water regulation, enzyme activation, and nutrient transport.

A deficiency of potassium can lead to the development of yellow leaves, also known as chlorosis. The dead spots on the leaves can occur due to tissue death caused by inadequate potassium levels.

Let`s analyze the other options to understand why they are not the correct answers:

Option 2: Magnesium - Magnesium deficiency typically leads to interveinal chlorosis, which means the yellowing occurs between the veins of the leaves, rather than having dead spots.

Option 3: Nitrate - Nitrate deficiency usually causes stunted growth and general yellowing of the entire plant, rather than specific dead spots on the leaves.

Option 4: Phosphate - Phosphate deficiency may cause dark green or purple leaves, and not yellow leaves with dead spots.

Hence, based on the given symptoms, the correct answer is option 1: Potassium.