A) 17fgh has only one term. A term is an algebraic expression or just a number. It doesnt matter how large or small its value, and as long as you havent addd or subtracted from it
B) (14m) (13n) these are two terms. The parenthesis means both expressions would multiply each other. Also the m and the n have effectively separated both "terms." You can not add both because the value of m would always be different from that of n. You can only add them if you find the value of m and n. However, note that you can only do that if the plus sign (or the minus sign) has been introduced. In this case it hasnt, so both terms can only multiply each other.
C) a - b + 13c has three terms. The letters a, b and c makes it three different values. Only variable c has a coefficient higher than 1 (that is 13 is its coefficient). The variables a and b both have a coefficient of 1. So option C is the correct answer.
D) 3x + 2yz also has only two terms