Which one is the singular form?

The correct answer is - গ) radius.

• কিছু কিছু noun এর plural অনিয়মিতভাবে হয়ে থাকে। যেমন:

Singular --------- Plural
alumnus --------- alumni
medium --------- media
radius ------------ radii
syllabus ---------- syllabi, syllabuses

• More Examples
Singular --------- Plural
basis ------------- bases
crisis ------------ crises
memorandum -- memoranda
vertex ------------ vertices, vertexes
formula --------- formulae, formulas
oasis ------------- oases
agendum -------- agenda
analysis ---------- analyses

Source:
1. A passage to the English Language by S.M. Zakir Hussain.
2. Applied English Grammar and Composition by P. C. DAS.