The plural form of 'Crisis' is-

• Crisis (noun)
Plural: Crises.
English Meaning: the turning point for better or worse in an acute disease or fever.
Bangla Meaning: সংকটকাল; চূড়ান্ত পর্যায়; সন্ধিক্ষণ।

• কিছু কিছু noun এর plural অনিয়মিতভাবে হয়ে থাকে। যেমন:
Singular: Radius - Plural: Radii
Singular: Crisis - Plural: Crises
Singular: Thesis - Plural: Theses
Singular: Medium - Plural: Media

Singular: Agendum - Plural: Agenda
Singular: Datum - Plural: Data
Singular: Bacterium - Plural: Bacteria
Singular: Focus - Plural: Foci

Singular: Criterion - Plural: Criteria
Singular: Alumnus - Plural: Alumni
Singular: Fungus - Plural: Fungi

Singular: Cactus - Plural: Cacti (or Cactuses or Cactus)
Singular: Octopus - Plural: Octopi (or Octopuses)
Singular: Index - Plural: Indices (or Indexes)
Singular: Appendix - Plural: Appendices (or Appendixes)

Source:
1. A Passage to the English Language by S.M. Zakir Hussain.
2. merriam-webster.