Market Order method will place the order at the best price available on the market.
The current price is 1500. If you place a market order, your order will be placed at the last price. This means that the order is not necessarily placed at 1500 (A), but may be slightly lower or higher than 1500 depending on market liquidity.
For buy orders, the average price filled may be slightly higher than the current price. For sell orders, the average price filled may be slightly lower than the current price.