In this question, one term in the number series is

Select the option that can be used as a one-word substitute for the given group of words.

  A herd or flock of animals being driven in a body

Select the ANTONYM of the given word.

FINITE

Give a single word that substitutes for the following expression:

A speech delivered without prior preparation

Select the option that can be used as a one-word substitute for the given group of words.

Many fish swimming together

Unanimously

Select the option that can be used as a one-word-substitute for the given group of words.

A person who studies insects:            

Select the option that can be used as a one-word substitute for the given group of words.

Select the most appropriate option to substitute the highlighted segment. If no substitution is required select 'No substitution'.