挺 as an adverb means: very; rather; pretty; quite
我看你红光满面,___精神的。
I see your face is glowing with healthy color, you seem ___ strong and spirited.
very; rather; pretty; quite are all correct answers.
好精神的 is incorrect in this context because 好 is the colloquial form of 很(very). 我看你红光满面,很精神的。would be grammatically correct.
However, the sentence in the question ended with a period, indicates the sentence is completed. Which means the first part "我看你红光满面" is the observation, and the second part "挺精神的" is the estimation (a conclusion from the observation), 很 sounds too definite
我看你红光满面, 太精神的 (too strong and spirited) makes not much sense.
我看你红光满面, 多么精神的 (how strong and spirited it is) '多么' usually pair with an finish particle '啊' not '的'