(a) No, that's not the best answer. It does not particularly matter whether a business venture involves a single undertaking or not. What really distinguishes a joint venture from a partnership is that partners combine their assets and skills together to run a business in common. Joint venturers are each involved separately in a business venture to achieve one or more specified outcomes.
The persons who engage in a joint venture normally agree what they will each contribute, either in capital, assets, expertise or work. They will also need to agree on their respective entitlement to what is produced or achieved. But in a joint venture, each party operates as an individual, and each party bears responsibility on their own for expenses and losses.
In the present case, it appears that the arrangement between E&R Plumbing and Domingo would be a joint venture in which they or their individual business organisations participate separately.
It should be noted that the courts have held that the term 'joint' venture' is not a technical term that always has an unambiguous legal meaning. Sometimes, because of the underlying facts, the courts have found that what the parties have called a joint venture is in a fact of partnership, a trust, an agency or a joint ownership. In such cases, the rights and duties of the parties are determined accordingly.