chaitanya.mehrotra wrote:In the below examples how come both the mentioned options are correct for each of the sentences . I though all of the below collective nouns must always take singular verbs .

Half of my family lives/live in Canada. ( Family --> Lives )
All of the class is/are here. ( Class --> Is)
Ten percent of the population is/are bilingual.( Population--> Is)

Question: In " half of my family" , "all of the class", "Ten percent of the population" , how do we decide the subject and whether its plural or singular?

you are correct; those should be singular.

if you have a source claiming that they can be either way, then the source is incorrect. (the source may be correct by the standards of other dialects of english, such as british english -- but those dialects are irrelevant to the gmat, which strictly tests standard american english.)

so, you should mark whatever source this came from as unreliable.

These are from SC resources from beat the gmat site itself

the majority of "resources" that i've seen posted by random members are, in a word, awful.

in general, if a gmat verbal document is not produced by a reputable company, it's almost certainly going to be unreliable, and it will often be objectively wrong about many, many things.
as my uncle used to say ... "things that you get for free are worth exactly what you paid for them."