What to capitalize in table headings?

Solution 1:

Chicago Manual of Style, the style guide which journal styles in the mathematical and physical sciences are most often based on —including the IEEE, the most widely followed style guide in computer science— follows your option #2, "Programming language".

You don't say what style you are writing to: If you are planning to submit it to a journal or as part of a book, the publisher's guidelines for authors should have sufficient information for you to decide. If you are submitting it as part of degree coursework, your university might or might not have a rule here. Otherwise, consider yourself bound only by the requirement to be consistent.

Solution 2:

As far as I have experienced it, capitalisation of headings is very much dependent on the culture you live in. In US English, almost every word in a heading is capitalised. In British English, I see much less capitalisation. In NZ, my country of residence, it's perfectly all right to just capitalise the first word of a heading, and you'd probably also get away with option 3.

The above mentioned recommendations to find out about the style guide for the particular publication are probably the best advice you can get.

Solution 3:

I think it's the first one (also around you'll see it's the used one), but don't use the third one because it will look bad, no matter the type of table.

There are some rules, anyway, about what you can capitalize and what not, i.e. no function words (but, not, of). Check here (PDF).