Comparing proportions I have a sample of 65 participants.
20 chose A.
18 chose B.
14 chose C and 
13 chose D.   What is the suitable test to determine if there is significant difference in the proportions?
 A: It's not completely clear that I have understood correctly, but I think you're asking whether the proportions choosing A, B, C and D are consistent with the population proportions being $(\frac{1}{4},\frac{1}{4},\frac{1}{4},\frac{1}{4})$.
If that's the case, this is just a chi-squared goodness of fit test. 
(Searching on chi-squared goodness of fit test or chi-square goodness of fit test will turn up many hits here, a few of which will relate to your kind of problem.)
Since this looks like the particular numbers may be from a self-study question, I won't do the problem itself, but I will explain the calculation of the test statistic.
Given the 65 participants you compute the expected count in each cell, $E_i = 65/4,\, i=1,2,3,4$. The observed counts, $O_i$ are given in your question.
You then compute the chi-square statistic, $X^2 = \sum_{i=1}^4 \frac{(O_i-E_i)^2}{E_i}$, which under the null hypothesis has a $\chi^2$ distribution with three degrees of freedom.
Most stats packages can do this work for you, but it's a few moments to do by hand if you have chi-square tables.
