Final answer:
An 18-month-old is most likely to say, "Dog park, please?" when requesting to take the dog to the park, as it reflects the typical two-word phrase capability of children at this age.
Step-by-step explanation:
The phrase most likely to come from an 18-month-old and mean "Mother, I would like to take the dog to the park" is (a) "Dog park, please?". At 18 months, children are beginning to put two-word phrases together and can understand simple requests and instructions. The options (c) "Want dog, park!" and (b) "Dog want park!" are possible utterances from a toddler but are a bit less likely because they involve a slightly more complex structure. Option (d) "I want to take the dog to the park" is far too complex for typical 18-month-old speech development.