Currency options

Assuming they are traded options, then yes. You can buy and sell both call options and put options.

When do I sell call options?

In the exam you won't - you will either exercise them or not. However European style options can only be exercised on the last day of the relevant month and so if the underlying transaction is not at the end of the month, then in 'real life' you would sell the option on the date of the transaction (if the price had increased).