Final answer:
According to Newton's first law, if no other forces act on a moving object, its mass will remain constant. While speed can also remain constant under these conditions, mass specifically is a fundamental characteristic that does not change without external influence. Therefore, the correct answer to the question is mass.
Explanation:
Understanding Newton's First Law
According to Newton's first law of motion, also known as the law of inertia, an object will remain at rest or in uniform motion in a straight line unless acted upon by a net external force. This means that if there are no net external forces acting on a moving object, certain characteristics of that object will remain constant.
In the context of the options given in your question, the characteristic that would remain constant is mass. Mass is a measure of the amount of matter in an object and does not change unless the object loses or gains material. For example, a car moving at a constant speed of 60 km/h on a straight road will maintain its mass as it continues moving, provided no external forces, such as friction or air resistance, are significant enough to affect its motion.
While the object's speed might stay constant if no forces are acting on it, it is important to understand that speed refers to the rate at which it covers distance, which could change if forces were applied. Conversely, the size and shape of an object can also remain constant, but they are not directly related to the motion described by Newton's law.
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