Vectors a and b are collinear iff
Aa · b = 0
B|a| = |b|
Ca × b = 0
Da + b = 0
Answer & Solution
Correct answer: C. a × b = 0
1. Two vectors are collinear (parallel or anti-parallel) when sin θ = 0.
2. Then |a × b| = |a||b| sin θ = 0, i.e. a × b is the zero vector.
3. Hence the condition a × b = 0.
_Source: Maharashtra Balbharati Std XII Mathematics Part 1, Ch 5 "Vectors" §5.5_
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