Update Salesforce Opportunity (not contact/account) with Task informing Email Was Sent out of Marketo

Unfortunately Marketo can only read the opportunity object, so achieving this out of the box is not possible.

if the data value change is coming from the opportunity you're trying to create a task on, I can think of a way to achieve this by using trigger smart campaigns with velocity's special triggerObject allowing you to send an alert with the opportunity ID (documentation found here https://developers.marketo.com/email-scripting/marketo-objects/#trigger_objects) (I can't see opportunityId in my list of fields so you might need to create a custom formula field on the opportunity object in Salesforce that copies the opportunity ID). However, you'll need to use some kind of email parser to parse the email and follow up with a webhook/api into SFDC to update the record.

I just remembered something. You can send an alert of the email to the email-to-salesforce address of your sync user and add REF: <opportunityId> to the subject line, which salesforce will create an activity on the record with the body of the email. SFDC documentation found here: https://help.salesforce.com/articleView?id=email_my_email_2_sfdc_using.htm&type=5

You'll need to have 2 copies of the email, one that goes out to the recipient and one that goes to salesforce (so you can modify the subject line). You'll also need to whitelist sender email addresses in your email-to-salesforce settings found in my settings