The father sat vigilantly beside the sick baby. Here the underlined part is -

• The father sat vigilantly beside the sick baby. Here the underlined part is - Adverb.

• এখানে, 'vigilantly' adverb 'sat' verb- কে modify করেছে।
- অর্থাৎ, যে word কোনো verb- কে modify করে, তাকে Adverb বলে।

• Vigilantly (adverb)
English Meaning: in a way that is always careful to notice things.
Bangla Meaning: সতর্কভাবে।

Example Sentence: Our community has acted vigilantly to ensure that all activities are appropriate.

Source: Cambridge Dictionary.