Answer the question truthfully. Although you were not convicted for the offense you were placed on probation. My assumption is adjudication was withheld which translates to no formal conviction. There is a possibility this will exclude you from employment but they will definitely be able to find it so the best policy is to be truthful.
Darren is correct. The federal government does not recognize no contest pleas or withheld adjudications and considers them to be convictions for the purposes of the question they ask. They can, and probably will, find any that you have and if you lied, you may be facing a perjury charge. Tell the truth.
I have had many prospective clients tell me, after being rejected for a job, that they did not list prior crimes because adjudication was withheld or they otherwise believed that they were being honest on their applications when they said they had no convictions. Most employers can forgive prior indiscretions...what they cannot forgive is dishonesty. In other words, they often don't care what you did; they care if you lie about it. List your priors and give a short explanation of your plea and sentence as best you can, if the form provides space for an explanation. Simple online searches by employers will give them the information anyway so I always advise clients to be honest.