letter c in Lithuanian

Press and hold the letter 'c'. You should see a pop-up just above with an indexed row of alternate representations of the letter 'c'. On my U.S. English extended keyboard, using the U.S. entry from the keyboard viewer menu in the menu bar — I get the following:

One can then type 3, or simply click on the third item to replace the letter 'c' with that accent. The preceding was done in Pages v8.0 with the Edit menu : Substitutions : Text Replacement selected.

It's not available in the English keyboard: I can't answer for other languages. Open System Preferences>Keyboard and check 'Show Keyboard a & Charcter Viewrs in menu bar'. Click on the icon in the menu bar and choose 'Show keyboard viewser'. Press the alt key to see whether this diacritical is available.

Failing that, open the Character Viewer and choose the Latin section:

Double-click the character to insert it.