**Weegy:** the answer is
for

**User:** An equation _____ has one solution.

**Weegy:** It's either a trick question, [ or the person who wrote the question is guilty of fuzzy thinking.
a linear (1st order) equation will have one solution
a quadratic (2nd order) equation will have two solutions
a cubic (3rd order) equation will have three solutions
...
an nth order equation will have n solutions.
An equation may have more than one solution, or the solution(s) may be complex, rather than real, but it will always have one solution - and sometimes more than one.
Had the question been "An equation _____ has only one solution", the answer would have been #2, "sometimes". ]

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