This is a vocabulary and linguistics question, and the answer is homonyms. When two words are written exactly the same way and spoken exactly the same way, but their meanings do not overlap at all, English grammar labels that pair as homonyms. The example given, "left" as a direction versus "left" as the past form of "leave," fits this definition perfectly since both spelling and sound match while meaning does not.
The other three terms describe different relationships between words. Synonyms share meaning, not spelling or sound, so pairs like "big" and "large" fall under that category instead. Heteronyms are words identical in spelling but different in pronunciation, the reverse case, such as "tear" (to rip) versus "tear" (a drop from crying). Acronyms are shortened forms built from the first letters of a longer name or phrase, such as "RADAR," which is an entirely separate concept from matching spelling and sound. That confirms homonyms as the right term.
