The plural form of "crisis" is -

The correct answer is - খ) crises

কিছু কিছু noun এর plural অনিয়মিতভাবে হয়ে থাকে। যেমন:
Singular --------- Plural
crisis ------------ crises
syllabus ---------- syllabi, syllabuses
vertex ------------ vertices, vertexes
basis ------------- bases
crisis ------------ crises
memorandum -- memoranda

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