Which of the following words is in singular form?

♦ Radius: [singular form]
English meaning: a straight line joining the center of a circle to its edge or the center of a sphere to its surface.
Bangla meaning: ব্যাসার্ধ।
Plural form: radii.

Other options,
• Phenomenon হচ্ছে singular, যার plural হচ্ছে Phenomena 

• Agendum: [singular]
- Meaning: সভার কার্যতালিকা।
- Plural form: agenda.

• Louse (উকুন) শব্দটি singular এর plural form হবে Lice.

Source: Accessible & Cambridge Dictionary.