Choice displays appropriate use of parallel structure and correct use of commas?

a) to boat, to ski, and sunbathing are three activities in which a person can take part at the lake.
b) to boat, to ski, and to sunbathe, are three activities in which a person can take part at the lake.
c) to boat, skiing, and to sunbathe, are three activities in which a person can take part at the lake.
d) boating, waterskiing, and sunbathing are three activities in which a person can take part at the lake.

D) Boating, waterskiing, and sunbathing are three activities in which a person can take part at the lake.

Explanation: Parallelism or parallel structure is a literary device that consists in the repetition of the grammatical structure of words or phrases in a text. In the given sentences we can see a list of elements, in order to them to be correct, they must be consistent in the form of the nouns and verbs (they should be all in the infinitive form or all in the gerund form), so the options B and D use parallel structure correctly, but option B have a comma before the verb which is incorrect (we should not use commas to separate the subject from the verb), so the correct answer is option D.  

i think the life is invaluable.

you can also say immeasurable.

i think it may be b, i'm not exactly too sure on this. but b seems to make more sense.