The short answer is Bid price.
However, even though this is the default, it is possible to also view the Ask price on your trading terminal.
To display the Ask price:
Right-click on the chart and select Properties
In the Properties window, click the Show tab
Tick Show Ask price line and then click Ok
However, since candles in MetaTrader 5 only include the bid price, there is no feature for displaying historic ask prices.