Which of the following words is in singular form?

• "Vertex" is a singular form of - 'Vertices'.

• "Vertex": 
- (noun). 
- (plural vertices). 
বাংলা অর্থ:
- শীর্ষ; শীর্ষবিন্দু; চূড়া।
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অন্যদিকে, 
"Syllabi" is the plural form of "syllabus."
"Oases" is the plural form of "oasis."
"Fungi" is the plural form of "fungus."

Source: Accessible Dictionary by Bangla Academy, merriam-webster Dictionary, Cambridge Dictionary.