For medical deductions it says "You have a choice on how to claim. You can either take the Standard Deduction or itemize. " How can you opt for standard deduction?

I have less than $6,000 in medical bills so I guess I got the standard deduction?

Medical expenses are only a part of the itemized deductions.  Also, your Adjusted Gross Income (AGI) is taken into account when determining whether any of your medical expenses are deductible.  They must exceed 7.5% of your AGI to be deductible.  

If you are not sure whether you are using the standard deduction or itemized deductions on your return, see the information in the following TurboTax article to learn how to compare your choices and switch between them:

How do I change from the standard deduction to itemized deductions and vice versa?

see this info from Turbotax. There is a limit to medical expenses deductions. Only the amount over 7.5% of your income will be deductible https://turbotax.intuit.com/tax-tips/health-care/can-i-claim-medical-expenses-on-my-taxes/L1htkVqq9

  Items that are included in the Itemized dedns vs Standard deduction are state taxes, mortgage interest, charitable deductions and medical expenses. Turbotax program will automatically choose the higher amount for you when you complete your information. 

  If you are using the "Live" version as your question shows here is how to contact the experts

I have only $400 in medical expenses. Can i claim this with standard deduction

No.