Answer: Choice D) 3n^2
"mono" means "one". I often think of "monorail" which means "one rail" to help remember this. So "monomial" means "one term". This reduces our choices to either C or D, as they show one term each. Choices A and B are ruled out as these are binomials, showing two terms each.
Choice C shows a cubic monomial since the exponent here is 3. So the degree is 3. We can rule out choice C.
Choice D has a 2nd degree monomial because the exponent is 2. The leading coefficient is 3 as this is the number to the left of the variable term. All of choice D fits with the description of "A monomial of the 2nd degree with leading coefficient of 3"