Select the most appropriate option to substitute the underlined segment in the given sentence. If there is no need to substitute it, select ‘No substitution required.’

A tigress has given birth to a cub in the Ranthambore Tiger Reserve, taking the big cat population to 78.

The correct answer is No Substitution required.

Let's analyze the grammatical structure of the underlined segment:
The subject of the sentence is "A tigress", which is singular.
The verb phrase "has given birth" uses the present perfect tense (has + past participle given) of the idiom "give birth".
This tense is appropriate here because it describes a recently completed action in the past that has direct relevance to the present (resulting in the big cat population increasing to 78).

Let's evaluate the other options to understand why they are incorrect:
1. is given births: This is grammatically incorrect. The verb form is passive and the plural "births" does not fit the context of a single cub.
2. was birthed: While "birth" can sometimes be used as a verb, "gave birth to" or "has given birth to" is the standard, natural active phrasing. Since the original active construction is correct, substitution is not needed.
3. has give birth: This is grammatically incorrect. The present perfect auxiliary verb "has" must be followed by the past participle form of the verb ("given"), not the base form ("give").

Therefore, the original sentence is grammatically correct and requires no substitution.