If a margin call is not met, your broker can liquidate whatever they want. They usually do not give you a choice.
From TD Ameritrade Helpdesk:
When is my margin call due?
Regulation T calls must be met within five (5) business days, without exception. Margin Maintenance calls, and equity calls are due immediately, and liquidation could occur at any time, due to market conditions, even without prior notice.
IMPORTANT REMINDER: MARKET CONDITIONS COULD DICTATE A FORCIBLE SELLOUT IN YOUR ACCOUNT WITH OR WITHOUT NOTIFICATION, REGARDLESS OF YOUR INTENT TO COVER THE AMOUNT DUE. FORCIBLE LIQUIDATIONS MAY RESULT IN A SELLOUT FEE IN ADDITION TO THE APPLICABLE COMMISSION CHARGES.
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