Of these diacritical marks, which one appears below a letter, rather than above it?

A cedilla (from Spanish) or cedille (from French 'cédille') is a hook or tail ( ¸ ) added under certain letters as a diacritical mark to modify their pronunciation. In Catalan, French, and Portuguese (called cedilha) it is used only under the c (forming ç), and the entire letter is called, respectively, 'c trencada' (i.e. "broken C"), 'c cédille', and 'c cedilhado' (or 'c cedilha', colloquially). It is used to mark vowel nasalization in many languages of sub-Saharan Africa, including Vute from Cameroon.

A diacritic (also diacritical mark, diacritical point, diacritical sign, or accent) is a glyph (purposeful mark) added to a letter or to a basic glyph. The circumflex, tilde and umlaut all have diacritical mark that appear above the letter.