Which one of the following is formed (mainly) when red phosphorus is heated in a sealed tube at 803 K?

First, recall the well-established experimental facts about the allotropes of phosphorus.

Black phosphorus exists in two crystalline modifications that chemists name $$\alpha$$-black and $$\beta$$-black. We have the following preparation conditions:

$$\alpha\text{-black P} : \text{obtained by heating red phosphorus in a sealed (evacuated) tube at } T \approx 803\ \text{K}$$

$$\beta\text{-black P} : \text{obtained by heating white or }\alpha\text{-black phosphorus at a lower temperature }(473\text{-}523\ \text{K})\text{ BUT under very high pressure (}\gtrsim 1.2\times10^{4}\text{ atm)}.$$

Now, the question specifies red phosphorus heated in a sealed tube at 803 K. There is no mention of any externally applied high pressure. So the experimental conditions exactly match the first line written above.

Therefore red phosphorus rearranges into the layered, lustrous form called $$\alpha$$-black phosphorus under these conditions.

Hence, the correct answer is Option A.