• The plural word is -Spectra.

• Spectrum [Singular]
- English meaning: A band of colours, as seen in a rainbow, produced by separation of the components of light by their different degrees of refraction according to wavelength.
- Bangla meaning: বর্ণচ্ছটা; বর্ণালি; (লাক্ষণিক) বিস্তৃতি, বিস্তৃত পরিসর বা ধারা।

- Plural form: Spectra, Spectrums.

• অন্যদিকে,
ক) Analysis [Singular form]
- English Meaning: a detailed examination of anything complex in order to understand its nature or to determine its essential features: a thorough study.
- Bangla Meaning: মন্তব্য সহকারে (পুস্তক, কোনো চরিত্র বা পরিস্থিতির) বিশ্লেষণ:

- Plural form: Analyses.

গ) Index: [Singular form]
- English meaning: an alphabetical list, such as one printed at the back of a book showing which page a subject, name, etc. is on.
- Bangla meaning: সূচক; দেশক; নির্দেশক।

- Plural form: Indices, Indexes.

ঘ) Child [Singular form]
- English Meaning: a young person especially between infancy and puberty.
- Bangla Meaning: অজাত বা নবজাত মানবশিশু; বালক বা বালিকা; (যেকোনো বয়সের) পুত্র বা কন্যা।

- Plural form: Children.

Source:
1. Accessible Dictionary by Bangla Academy.
2. Merriam-Webster Dictionary.